The Siemens SENTRON 3VA9978-0BA23 is a shunt trip — an auxiliary release that remotely trips a circuit breaker when voltage is applied. It's designed for the 3VA4 and 3VA5 (frames 1-4) and 3VA6 (frames 1-4) molded-case breakers. That's a healthy coil; make sure your control transformer can handle the surge.
Mechanically, this accessory snaps into the three designated slots on the breaker's side. Physical footprint is 31.2 mm deep, 21.7 mm wide, and 65.1 mm tall — it doesn't add width to the breaker's DIN-rail envelope, but it does project slightly beyond the breaker face. The terminals accept conductors with a 10 mm strip length.