SNAP DISC MANUAL RESET  also known as the Blower Manual Reset.

This sensor has a red reset button in the center for Earth Stove pellet stoves. It stops power flow in case of heating over 200 degrees and needs a manual reset to allow power to flow again. If this sensor is being triggered by overheating it suggests that the convection or room air blowers are clogged or failing. Dirty fan blades or a clogged inlet screen can cause this issue. This sensor is there to protect the control panel from overheating if the blowers are not cooling this area properly.

(old part # 15012 is an auto reset and is available) 

For  Earth Stove pellet stoves including - TP300, TP40, TP340, TP440, HP40, FP60, RP45, WP50, MP35-50, MP40, MP240, MP250, NF2000, NF2000I, PI35, Homewarmer and Traeger pellet stoves

Part number 15011 or alternative reference numbers -  60T15 or 330784 L   

shop notes - GR - 2E364 or 6uec4