The Siemens 5SL4203-6CC is a 2-pole miniature circuit breaker (MCB) rated at 3 A with a B-curve trip characteristic, breaking capacity of 10 kA at 400 V.
Snap-on DIN rail mounting, standard 2-pole width (roughly 36 mm on a 17.5 mm/pole module). The 3 A rating means it's sized for a 1.5 mm² copper conductor on the load side — typical for a lighting subfeed or a small control transformer primary. The B-curve avoids nuisance tripping on moderate inrush but won't hold a motor start; if you're switching a small motor, step up to a C-curve sibling in the same 5SL4 family.