References to other fonts to be used looking for fallbacks.
This is a list of references to other Font assets in your project, which will be used when looking for the fallback fonts listed in fontNames when trying to render a character which is not available in the font. If you set the fallback font names in the Font importer inspector in the editor, Unity will automatically search your project for matching assets and populate the references automatically - so you should only ever need to set this yourself when importing Font assets programmatically.
Additional resources: TrueTypeFontImporter.fontNames.