If you want to play OP (Organized Play) or STC social events, you will need an STC rating.
For tennis, if you have a current and active USTA rating, you can use that rating. If you do not have a current and active USTA rating, you will need to self-rate using the NTRP guidelines (available on the STC website) and validate that rating by completing the New Member Provisional Process. If you want to appeal your STC tennis rating you will also need to complete the Provisional Process. The process, forms, etc are all available on the website.
For POP, a complete procedure for determining POP ratings is under consideration. B (beginner), I (intermediate),and A (advanced) ratings are currently being used. If you are unsure what your rating should be, ask one of the STC teaching pros.