For example:
<extent>:
Required to be the first child of the <body> element.
<feature>, <tile>:
Child of the <body> element.
<meta>:
In the <head> element.
<link>:
If the element represents a hyperlink (has a href attribute): as a child of the head or body element.
The above should only be true if the root document is MapML. I believe all these elements can be children of the <layer> element as well.