[IMP] Taxes: improve tax configuration doc

task-5094417

We improve the following sections of the Taxes page:

- Tax computations:
  At the moment, the section for the `Percentage of Price tax included`
  tax computation doesn't distinguish itself enough from the 'Included
  in Price' option which people usually want.
  We also improve the examples.

- Included in Price
- Affects base of subsequent taxes
- Base affected by preceding taxes: this option was not documented, so
  we add a section for it.

Author: antoine162

content/applications/finance/accounting/taxes.rst

@@ -72,23 +72,40 @@ The **tax name** is displayed for backend users in the :guilabel:`Taxes` field i
 Tax computation
 ~~~~~~~~~~~~~~~
 
-- **Group of Taxes**
+The :guilabel:`Tax Computation` field determines how the tax amount is computed from the sales
+price. The following options are available:
+
+- :ref:`taxes/computation/group-of-taxes`: a combination of several other taxes
+- :ref:`taxes/computation/fixed`: a fixed amount
+- :ref:`taxes/computation/percentage-of-price`: a percentage of the tax-excluded sales price
+- :ref:`taxes/computation/percentage-of-price-tax-included`: a percentage of the tax-included total
+- :ref:`taxes/computation/python-code`: a custom user-defined formula.
+
+.. _taxes/computation/group-of-taxes:
+
+Group of Taxes
+**************
 
   The tax is a combination of multiple sub-taxes. You can add as many taxes as you want, in the
   order you want them to be applied.
 
-  .. important::
-     Make sure that the tax sequence is correct, as the order in which they are may impact the
-     taxes' amounts computation, especially if one of the taxes :ref:`affects the base of the
-     subsequent ones <taxes/base-subsequent>`.
+.. important::
+   Make sure that the tax sequence is correct, as the order in which they are may impact the
+   taxes' amounts computation, especially if any of the taxes :ref:`affects the base of the
+   subsequent ones <taxes/base-subsequent>`.
 
-- **Fixed**
+.. _taxes/computation/fixed:
 
-  The tax has a fixed amount in the default currency. The amount remains the same, regardless of the
-  sales price.
+Fixed
+*****
+
+The tax has a fixed amount in the default currency. The amount remains the same per unit,
+regardless of the sales price.
+
+The computation is :math:`\text{tax amount} = \text{fixed tax amount} \times \text{quantity}`.
 
 .. example::
-   A product has a sales price of $1000, and we apply a $10 *fixed* tax. We then have:
+   A product has a sales price of $1000, and we apply a $10 :guilabel:`Fixed` tax. We then have:
 
    +-------------+-------------+----------+----------+
    | Product     | Price       | Tax      | Total    |
@@ -97,46 +114,136 @@ Tax computation
    | 1,000       | 1,000       | 10       | 1,010.00 |
    +-------------+-------------+----------+----------+
 
-- **Percentage of price**
+.. _taxes/computation/percentage-of-price:
 
-  The *sales price* is the taxable basis: the tax amount is computed by multiplying the sales price
-  by the tax percentage.
+Percentage of Price
+*******************
 
-.. example::
-   A product has a sales price of $1000, and we apply a *10% of Price* tax. We then have:
+The tax rate is a percentage of the **tax-excluded** subtotal.
 
-   +-------------+-------------+----------+----------+
-   | Product     | Price       | Tax      | Total    |
-   | sales price | without tax |          |          |
-   +=============+=============+==========+==========+
-   | 1,000       | 1,000       | 100      | 1,100.00 |
-   +-------------+-------------+----------+----------+
+The exact tax computation depends on the :ref:`taxes/included-in-price` field, which determines
+whether the sales price should be treated as tax-excluded or tax-included:
+
+.. tabs::
+   .. tab:: Tax-excluded
+
+      If :guilabel:`Included in Price` is unchecked, the computation is :math:`\text{tax amount}
+      = \text{sales price} \times \text{tax rate}`.
+
+      .. example::
+         A product has a sales price of $1000, and we apply a 10% :guilabel:`Percentage of Price`
+         tax that is not :guilabel:`Included in Price`. We then have:
+
+         +-------------+-------------+----------+----------+
+         | Product     | Price       | Tax      | Total    |
+         | sales price | without tax |          |          |
+         +=============+=============+==========+==========+
+         | 1,000       | 1,000       | 100      | 1,100.00 |
+         +-------------+-------------+----------+----------+
+
+   .. tab:: Tax-included
+
+      If :guilabel:`Included in Price` is checked, the computation is :math:`\text{tax amount} =
+      \text{sales price} \times \frac{\text{tax rate}}{1 + \text{tax rate}}`.
+
+      .. example::
+         A product has a sales price of $1000, and we apply a 10% :guilabel:`Percentage of Price`
+         tax that is :guilabel:`Included in Price`. We then have:
+
+         +-------------+-------------+----------+----------+
+         | Product     | Price       | Tax      | Total    |
+         | sales price | without tax |          |          |
+         +=============+=============+==========+==========+
+         | 1,000       | 909.09      | 90.91    | 1,000.00 |
+         +-------------+-------------+----------+----------+
+
+.. _taxes/computation/percentage-of-price-tax-included:
+
+Percentage of Price Tax Included
+********************************
+
+The tax rate is a percentage of the **tax-included** total.
+
+The exact tax computation depends on the :ref:`taxes/included-in-price` field, which determines
+whether the sales price should be treated as tax-excluded or tax-included:
 
-- **Percentage of Price Tax Included**
+.. tabs::
+   .. tab:: Tax-excluded
+      If :guilabel:`Included in Price` is unchecked, the computation is :math:`\text{tax amount}
+      = \text{sales price} \times \frac{\text{tax rate}}{1 - \text{tax rate}}`.
 
-  The **total** is the taxable basis: the tax amount is a percentage of the total.
+      .. example::
+         A product has a sales price of $1000, and we apply a 10% :guilabel:`Percentage of Price
+         Tax Included` tax that is not :guilabel:`Included in Price`. We then have:
+
+         +-------------+-------------+----------+----------+
+         | Product     | Price       | Tax      | Total    |
+         | sales price | without tax |          |          |
+         +=============+=============+==========+==========+
+         | 1,000       | 1,000       | 111.11   | 1,111.11 |
+         +-------------+-------------+----------+----------+
+
+         Note that the real tax rate in terms of the tax-excluded price is
+         :math:`\frac{111.11}{1000} = 11.111\%`.
+
+   .. tab:: Tax-included
+
+      If :guilabel:`Included in Price` is checked, the computation is :math:`\text{tax amount} =
+      \text{sales price} \times \text{tax rate}`.
+
+      .. example::
+         A product has a sales price of $1000, and we apply a 10%
+         :guilabel:`Percentage of Price Tax Included` tax that is :guilabel:`Included in Price`.
+         We then have:
+
+         +-------------+-------------+----------+----------+
+         | Product     | Price       | Tax      | Total    |
+         | sales price | without tax |          |          |
+         +=============+=============+==========+==========+
+         | 1,000       | 900         | 100      | 1,000.00 |
+         +-------------+-------------+----------+----------+
+
+         Note that the real tax rate in terms of the tax-excluded price is
+         :math:`\frac{100}{900} = 11.111\%`.
+
+.. warning::
+   This tax computation is rarely used and only useful in countries (e.g. Brazil, Bolivia) that
+   quote tax rates as a percentage of the tax-included total.
+   For the more common need to compute tax amounts from a tax-included price, use the
+   :ref:`taxes/computation/percentage-of-price` tax computation with the
+   :ref:`taxes/included-in-price` option.
+
+.. _taxes/computation/python-code:
+
+Python Code
+***********
+
+A tax defined as :guilabel:`Python Code` consists of two snippets of Python code that are executed
+in a local environment that can access the unit price, quantity, product, and partner.
+:guilabel:`Python Code` defines the amount of the tax, and :guilabel:`Applicable Code` defines
+whether the tax is applied. Enter a formula for each field at the bottom of the
+:guilabel:`Definition` tab.
 
 .. example::
-   A product has a Sales Price of $1000, and we apply a *10% of Price Tax Included* tax. We then
-   have:
+   A product has a sales price of $1000, and we apply a :guilabel:`Python Code` tax with the
+   following configuration:
+
+   - :guilabel:`Python Code`: `result = price_unit * 0.10` and
+   - :guilabel:`Applicable Code`: `result = True`.
+
+   We then have:
 
    +-------------+-------------+----------+----------+
    | Product     | Price       | Tax      | Total    |
    | sales price | without tax |          |          |
    +=============+=============+==========+==========+
-   | 1,000       | 1,000       | 111.11   | 1,111.11 |
+   | 1,000       | 1,000       | 100      | 1,100.00 |
    +-------------+-------------+----------+----------+
 
-- **Python code**
-
-  A tax defined as **Python code** consists of two snippets of Python code that are executed in a
-  local environment containing data such as the unit price, product or partner.
-  :guilabel:`Python Code` defines the amount of the tax, and :guilabel:`Applicable Code` defines if
-  the tax is to be applied. The formula is found at the bottom of the :guilabel:`Definition` tab.
-
-.. example::
-   :guilabel:`Python Code`: `result = price_unit * 0.10`
-   :guilabel:`Applicable Code`: `result = true`
+.. warning::
+   In general, if a tax can be expressed as a multiple of the sales price or of the product
+   quantity, it is preferable to define it as a :ref:`taxes/computation/percentage-of-price` or
+   :ref:`taxes/computation/fixed` tax.
 
 .. _taxes/active:
 
@@ -256,24 +363,27 @@ invoice line.
 Included in price
 ~~~~~~~~~~~~~~~~~
 
-With this option activated, the total (including the tax) equals the **sales price**.
+With this option activated, the tax will treat the sales price on which it is applied as a total
+including the tax amount. The tax computation will split the sales price into a base amount and a
+tax amount. This makes it suitable for B2C sales in most countries where prices are quoted
+tax-inclusive.
 
 `Total = Sales Price = Computed Tax-Excluded price + Tax`
 
 .. example::
-   A product has a sales price of $1000, and we apply a *10% of Price* tax, which is *included in
-   the price*. We then have:
+   A product has a sales price of $1000, and we apply a 10% :guilabel:`Percentage of Price` tax
+   with :guilabel:`Included in Price` checked. We then have:
 
    +-------------+-------------+----------+----------+
    | Product     | Price       | Tax      | Total    |
    | sales price | without tax |          |          |
    +=============+=============+==========+==========+
-   | 1,000       | 900.10      | 90.9     | 1,000.00 |
+   | 1,000       | 909.09      | 90.91    | 1,000.00 |
    +-------------+-------------+----------+----------+
 
 .. note::
-   If you need to define prices accurately, both tax-included and tax-excluded, please refer to the
-   following documentation: :doc:`taxes/B2B_B2C`.
+   For a guide on configuring tax-excluded and tax-included prices for B2B and B2C customers,
+   see :doc:`taxes/B2B_B2C`.
 
 .. note::
    By default, only the :guilabel:`Tax excluded` column is displayed on invoices. To display the
@@ -287,8 +397,32 @@ With this option activated, the total (including the tax) equals the **sales pri
 Affect base of subsequent taxes
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-With this option, the total tax-included becomes the taxable basis for the other taxes applied to
-the same product.
+If this setting is checked, any subsequent tax applied on the same product line that has
+:ref:`taxes/base-affected` will "see" a modified sales price. The exact behaviour depends on
+whether the tax with :guilabel:`Affect base of subsequent taxes` is :ref:`taxes/included-in-price`
+or not.
+
+.. tabs::
+   .. tab:: Tax-excluded
+      If :guilabel:`Included in Price` is unchecked, subsequent taxes with :guilabel:`Base affected
+      by preceding taxes` will "see" a modified sales price equal to the tax-excluded price plus
+      the tax amount.
+
+      .. example::
+         A product has a sales price of $1000, and we apply a 10% :guilabel:`Percentage of Price`
+         tax with :guilabel:`Affect base of subsequent taxes`. Any subsequent tax with
+         :guilabel:`Base affected by preceding taxes` will "see" a modified sales price of $1100.
+
+   .. tab:: Tax-included
+      If :guilabel:`Included in Price` is checked, subsequent taxes with :guilabel:`Base affected
+      by preceding taxes` will "see" a modified sales price equal to the tax-included price minus
+      the tax amount.
+
+      .. example::
+         A product has a sales price of $1100, and we apply a 10% :guilabel:`Percentage of Price`
+         tax with :guilabel:`Included in Price` and :guilabel:`Affect base of subsequent taxes`.
+         Any subsequent tax with :guilabel:`Base affected by preceding taxes` will "see" a modified
+         sales price of $1000.
 
 You can configure a new :ref:`group of taxes <taxes/computation>` to include this tax or add it
 directly to a product line.
@@ -307,6 +441,17 @@ directly to a product line.
    .. image:: taxes/list-sequence.png
       :alt: The taxes' sequence in Odoo determines which tax is applied first
 
+.. _taxes/base-affected:
+
+Base affected by preceding taxes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This setting determines whether any previous tax with :ref:`taxes/base-subsequent` will modify the
+sales price "seen" by this tax.
+
+.. note::
+   Taxes with :ref:`taxes/included-in-price` always behave as if this setting is checked.
+
 Extra taxes
 ===========