The ODF format supports [1] setting more than just a single font, but rather setting a list of fallback fonts so that if an item on the list is not available  with the appropriate features on the system (e.g. no such family at such size), then the next list item - the next font - is chosen.  This is intended to make font choices more robust for display on systems which don't have the same installed fonts as the author's.

This feature is similar to how font selection is defined in SVG files and in HTML files via CSS; see:

https://css-tricks.com/css-basics-fallback-font-stacks-robust-web-typography/

This bug is intended to track the various internal logic and UI issues stemming from the use of these lists and the need to support it in LibreOffice.

NOTE: This is _not_ the same mechanism as global font substitution (bug 103342), though they are related.



[1] https://docs.oasis-open.org/office/OpenDocument/v1.3/OpenDocument-v1.3-part3-schema.html#attribute-svg_font-family ; and that directs you to:
 https://www.w3.org/TR/2008/REC-CSS2-20080411/fonts.html#font-family