Add glossary and clarification on opaque strings and identifiers

Process.yml

@@ -987,10 +987,11 @@ $graph:
         secondary file from that expression.
 
         To work on non-filename-preserving storage systems, portable
-        tool descriptions should treat `location` as an opaque
-        identifier and avoid constructing new values from `location`,
-        but should construct relative references using `basename` or
-        `nameroot` instead, or propagate `location` from defined inputs.
+        tool descriptions should treat `location` as an
+        [opaque identifier](#opaque-strings) and avoid constructing new
+        values from `location`, but should construct relative references
+        using `basename` or `nameroot` instead, or propagate `location`
+        from defined inputs.
 
         If a value in `secondaryFiles` is a string that is not an expression,
         it specifies that the following pattern should be applied to the path

concepts.md

@@ -56,6 +56,13 @@ compatibility.  Portable CWL documents should not rely on deprecated behavior.
 Behavior marked as deprecated may be removed entirely from future revisions of
 the CWL specification.
 
+## Glossary
+
+<a name="opaque-strings"></a>**Opaque strings**: Opaque strings
+(or opaque identifiers, opaque values) are nonsensical values that are
+swapped out with a real value later in the evaluation process. Workflow
+and tool expressions **should not** rely on it nor try to parse it.
+
 # Data model
 
 ## Data concepts
@@ -538,11 +545,11 @@ be provided:
     must be 'null'.
   * `runtime`: An object containing configuration details.  Specific to the
     process type.  An implementation may provide
-    opaque strings for any or all fields of `runtime`.  These must be
-    filled in by the platform after processing the Tool but before actual
-    execution.  Parameter references and expressions may only use the
-    literal string value of the field and must not perform computation on
-    the contents, except where noted otherwise.
+    [opaque strings](#opaque-strings) for any or all fields of `runtime`.
+    These must be filled in by the platform after processing the Tool but
+    before actual execution.  Parameter references and expressions may only
+    use the literal string value of the field and must not perform computation
+    on the contents, except where noted otherwise.
 
 If the value of a field has no leading or trailing non-whitespace
 characters around a parameter reference, the effective value of the field

invocation.md

@@ -104,8 +104,8 @@ the ability to accept inbound connections.
 The `runtime` section available in [parameter references](#Parameter_references)
 and [expressions](#Expressions) contains the following fields.  As noted
 earlier, an implementation may perform deferred resolution of runtime fields by providing
-opaque strings for any or all of the following fields; parameter references
-and expressions may only use the literal string value of the field and must
+[opaque strings](#opaque-strings) for any or all of the following fields; parameter
+references and expressions may only use the literal string value of the field and must
 not perform computation on the contents.
 
   * `runtime.outdir`: an absolute path to the designated output directory