It depends on how your pool is plumbed. Although it's not indicated in your signature, it sounds like you have an automation system (Compool?) that switches the valves from POOL mode to SPA mode? Typically, when in POOL mode, 100% of your water is drawn from the pool, then most returns to the pool, but some returns to the spa to keep it filling & overflowing into the pool to keep the spa sanitized & filtered. This may be done by adjusting the valve actuator "stops" or perhaps by a bypass line or return line into the spa. You may have manual valves for both the waterfall and fountain which, if turned off, may stop all water flow into the spa. If you turn off the waterfall and fountain and the spa stops overflowing, then you'll have your answer. Just make sure that you make sure that you have at least one return line open at all times when the pump is running. There are many possible variations to this, but this is best guess with the information you've provided.
