Solarian isn't exactly a gender, it's better thought of as a gender alignment or gender presentation. It doesn't have to correspond to one's actual gender. There can be feminine men and masculine women. Solarian is for masculine non-binary people/non-binary people who wish to present masculinely. For some people it can be used as a gender on it's own, but other people use it as a word for presenation, similar to how butch or femme are used.