Think it is the -ve terminal. Between the battery -ve terminal and the clip on/off earth chassis lead is a current sensor (thick copper like looking piece) and to this is attached the sensor wire. It's purpose is to measure battery drain and charge to signal to the body computer the state of charge of the battery over time and the current (as now) current drain/load on the battery. From that info and other info the body computer decides if the complex start/stop conditions are OK to enable Start/Stop. The factors are quite complex, take into account cabin/external temperature, aircon setting, sunlight sensor info etc.