If you have a combination of a pool and spa, your automation panel will be programmed to isolate each body of water.
In POOL mode, water will be pulled from the pool and then returned to the pool AND spa. So the spa water will spill over into the pool, and the temperature of both bodies will be whatever the set "pool temp" is.
When you change to SPA mode, your automatic valves will turn and start pulling and returning from the spa ONLY. Your heater will also turn on to heat the water to the set "spa temp".