Some spas are set up to use the circulation pump as the booster pump.

Diverter valves would turn for suction from the spa drain to the circulation pump to filter to heater to spa jets with air venturi. In this configuration you would have a return to the spa but it would be used to divert some of the pool return for sanatized water to the spa then over the spillway. Do not set up your spa this way. Use a dedicated booster pump and blower with spa jets (example Waterway Poly Gunite Spa Jet). Do not put in pool return inlets as spa jets.

When you have dedicated booster pump and blower to the spa, you will still have pool return water diverted to the spa and over the spillway for sanitation.This setup is for shared circulation pump, filter, heater and sanatizer.
Look at this link for a simple pool spa diagram page 8
www.jandy-downloads.com/pdfs/6594.pdf