Standard Ligatures | Feature Liga

feature liga {

sub f f i by f_f_i;
sub f f l by f_f_l;
sub f f by f_f;
sub f i by fi;
sub f l by fl;

} liga;

Since all the rules in this feature are of the same type, they will be grouped in a single lookup.
Since no script or language keyword has been specified yet, the lookup will be registered for this feature under all the language systems.

Microsoft in its OpenType® specification explains the tag LIGA in such a way: “Replaces a sequence of glyphs with a single glyph which is preferred for typographic purposes. This feature covers the ligatures which the designer/manufacturer judges should be used in normal conditions.”

There are only 5 standard ligatures in Unicode.

uniFB00; uniFB01; uniFB02; uniFB03; uniFB04.

It’s not very reasonable to extend this quantity, because the standard ligatures should be active by default in most applications and if you put more ligatures in feature liga, it could lead to some problems (with spellcheckers, PDF files etc.).
Notice also that fi and fl ligatures do not adhere to the naming convention. The reasons are historical. Although you should have in mind Georg Seifert’s remark: “For final glyph names you should always use the AGLFN names and uniXXXX otherwise. There gives best compatibility (with PDF but also if someone needs to open your files)”.