Isaa,
The relay card is going to allow the app to turn the heater on and off, but it does not have the capability to select Pool or Spa.
In the big picture is does not matter, as I assume that you do not have an automation system, so you will be manually switching your valves from the Pool mode to the Spa mode and then back.
You would use the low current relay and run the 24 VAC from the heater into the line side of the relay. Then connect the load side of the relay back to the heater. When you select the relay in the app, the relay will close and allow the 24 VAC to go back to the heater, turning it on.
You will have to control the set point for the water temp at the heater.
If you have an automation system, then you should not need the relay card at all.
Thanks,
Jim R.