Depending what SS stands for various things can be called SS-category. E.g. if SS is a category then we talk about categories over SS, for them see overcategory.

This entry is rather about another notion of SS-category introduced in

which is similar to the notion of QQ-category of Rosenberg. It is called SS-category as it is suitable context for a generalization of a separable functor.

A little info is found in Toen: Homotopical and higher categorical structures in algebraic geometry. File Toen web unpubl hab.pdf. S-cats are closely related to Segal cats.

nLab page on S-category