While writing reports or manuscripts, researchers sometimes use the abbreviation fn, which implies

The abbreviation fn stands for footnote, a note placed at the bottom of a page.

It points the reader to an explanatory or citation note below the text.

Figure number is written as fig. or fig. no., not fn.

Following numbers is conveyed by ff., a different marker.

Final note has no standard scholarly abbreviation as fn.

Footnotes carry references, clarifications or side remarks off the main text.