SS is usually a classification within a race. In my area, SS is an open class, then there are beginner classes by age, sport by age, expert by age, and elite open class. So, your answer, for my area, is every mountain bike race includes a SS race.
Thanks for the clearification, but I was trying to convey the fact that, in the CPS series at least, there is a seperate classification for SS. It is also true that you will not be disqualified for riding an SS in any class.