STP - Spanning Tree Protocol is Layer 2 network Protocol that runs on Bridges and Switches.
STP supposed to be used to avoid loops on Data Link Layer of OSI model.
STP configuration you can find on Configuration -> Network page under Advanced section.
STP should be used in cases when AP and SMs are placed in same L2 broadcast domain because of some wrong configuration or any other reason.
STP can be enabled on AP or SM or both.
How to test STP on ePMP devices:
Send scores ofpackets from Laptop to broadcast IP address.(Arping tool can be used)
Try that scenario with STP off, - it will cause broadcast storm and even ePMP UI will be hardly reachable.
Then enable STP on AP or SM, and you will see any issues in broadcast domain.