A 10/13 series regular is a term that is used to describe a specific type of SR role.
Although a 10/13 role is still considered a series regular, they are only guaranteed to be in 10 (out of 13) episodes. 10/13s typically get paid less than an “all shows produced” (ASP) series regular since they are working less episodes. However, a 10/13 SR role is still considered a contracted role, meaning (in most cases), they become ineligible to go out for any other pilots/series.