The Sensata Dominican REP (or Mexico depending on where made) RP4A is a thermal protector* for lighting fixtures. It has a dedicated voltage of 277Vac and has low sensitivity. It has three leads, black, white, and red. When the temperature of the fixture is exceeded it will cause the light to blink. It is auto resetting for when the fault is corrected. The Sensata RP4A is designed to fit into a 1/2" KO.
*A thermal protector is a safety device designed to prevent lighting fixtures from overheating, which can lead to damage, fire hazards, and reduced lifespan of the fixture.