At https://orgmode.org/manual/Export-Settings.html#DOCF124 and (renumbered to "fn:123") in doc/org-manual.org, a foot- note is defined as "DEFINITION NOT FOUND":
| [tim@vagabond ~/src/org-mode]$ git grep -2 'DEFINITION NOT FOUND' -- doc/org-manual.org | doc/org-manual.org-this timestamp are exported. | doc/org-manual.org- | doc/org-manual.org:[fn:123] DEFINITION NOT FOUND. | doc/org-manual.org- | doc/org-manual.org-[fn:124] At the moment, some export back-ends do not obey this | [tim@vagabond ~/src/org-mode]$ The original footnote seems to have been added with commit e30aed8f0c62e74633f7a0398340f9bd7bdfa3cc (but personally I don't find that text very helpful).
