Description
With standardized emoji sequences, the author is responsible for the correct order of emoji characters, possibly mandatory variation selectors and zero-width joiners. For most cases and emoji input GUIs, this just works, though. If these sequences are not handled on the system level, Opentype fonts probably employ the rlig ‘required ligatures’ feature which is enabled (or enforced) by default.
It may be useful sometimes, for a reader or designer to control (additional) ligation. This would be handled by the liga or clig ‘contextual ligatures’ OTF features which can be enabled on demand.
Possible use cases: ligature aliases, e.g. Woman+Man+Child = Man+Woman+Child = Child+Man+Woman = Woman+Child+Man …, and non-standard ligatures, e.g. Police Car 🚓 + Woman 👩 = Police Woman = Police Officer 👮 + Female Sign ♀️.
#5 should be fixed first, of course.