The footnote (fn) element is used to nominate supplementary information that is not appropriate to be included in the flow of the content.

A footnote is usually seen as being specific to page layout output formats, so marking up text as a footnote could be viewed as recording presentational information within content; something that DITA tries to avoid.

However, footnotes can also be considered as semantic identifiers for supplementary information that doesn't belong inline in the text. The footnote content is not critical to understanding the content, but provides extra information that may be useful to some readers.

The footnote (fn) element is typically rendered in the output as a number in the flow of the text and the footnote content at the bottom of the page or topic. The footnote number is automatically calculated during the publishing process. In hypertext output formats, the inline footnote reference number may be hyperlinked to the footnote at the bottom of the topic. Some publishing tools may display the footnote content in a popup window (in some hypertext output formats).

An example of a footnote (fn) element is:
<p>Always wear a seatbelt.
  <fn>Some jurisdictions allow a medical exemption from wearing a seat belt
  in exceptional circumstances.</fn>