
transsexual is older and less used now than transgender. generally it's used because people are changing their sex to fit their gender and don't feel like they're changing genders. i've seen people use it to describe medically transitioning vs social transitioning, too. some people just don't feel comfortable with one or the other or don't feel respresented by the words. i've talked to non binary people who feel like they're changing gender but they're fine with their sex, for example, too
just jumping in to add that it’s not necessarily medically transitioning vs socially transitioning (though it can be). there is a contemporary resignification of transsexual occurring, it used to be pretty much exclusively for people who were surged up. the TS/TG gender wars of the 2000s/early 2010s were super contentious for this (and other) reason(s). a lot of the meaning of it really depends on how someone understanding queer/transfeminist history/theory.
there are trans people who medically transition who do not feel represented by the term, and there are trans people who haven’t/are unable to who do. it varies. there are other forms of temporary and/or permanent body modification that may contribute to this, such as hair cuts, hair removal, binding, tucking, vocal training, tattoos, piercings, etc.