TRSAs were the little brother to TCAs. Terminal Control Areas were regulatory with their very own weather minimums and rules (similar to Class B and at those very same airfields). TRSAs and TCAs were identical except that TRSAs were unregulated. They were there to provide guidance mostly to IFR traffic and radar service to anyone who asked. Today if you’re filed IFR you probably won’t notice any difference from Class C airspace but you don’t have to talk with them if you don’t want to if you’re VFR.