SFNT is a font-container format from Apple, originally designed to wrap TrueType fonts.
FreeType's subsystem for it contains a file with 3 functions that dereferences metrics data from the file, 1 of which might (depending on build flags) send some through a configured Font Variation's callbacks.
Similarly another file has a couple routines to lookup & free "bdf" properties from the fontfile.
Another file consults lookuptables both in the font & hardcoded to determine glyph names.
1/?