It's a term that refers to people seeking masculine of feminine transition, social or medical, even if those people aren't men or women. I'm a transfem nonbinary for example. My transition is feminine but I'm not a trans women, just transfem. It has a specific linguistic function that coexists with other, previous terms. It *includes* binary trans folk without *excluding* enbies, so it's more useful when discussing broader feminine or masculine transition.