The opera cattegorization also makes seven broad categorical voice types and not four like choral music. These are: for women- soprano, mezzo soprano, and contralto. For men- countertenor, tenor, baritone, and bass. As you can see, there are many different destinctions here that choral music doesn't make, including gender destinctions. There can be both male and female altos. The same is not true for contraltos as such roles on the operatic stage are played only by women in the history of opera. Different roles were written with castrato and countertenors in mind. The term "alto" in terms of range would cover the mezzo soprano and countertenor most closely. Contralto lies somewhere between the alto and tenor range in choral music.