footnote tag is used to generate a footnote on selected text.
This CNXML:
<para id="example-paragraph">
Many people may not know that cheesecake
is tart, not a cake.
<footnote id="example-footnote">
Alan Davidson, Oxford Companion to Food,
s.v. cheesecake: "Cheesecake, not a
cake in the ordinary sense, is really a
kind of tart."
</footnote>
</para>
Results in this display:
Many people may not know that cheesecake is tart, not a cake. 1