Internal temperature sensor for overheating protection. If the PCB temperature goes above a danger level for any reason, the device will turn off.
The relay will turn off the circuit immediately if it detects a consumption higher than the defined over-power setting.
Shelly 1PM comes with a programming/debug header which can be used to flash alternative firmware on the device. It has an ESP8266 inside, with a 2MB flash chip. A USB-to-UART adapter is needed as well as a reliable 3.3V with at least 350 mA drive capability.
The following diagram shows the device pinout: