The Siemens 5SM1312-6BA is a Type A residual current device (FI protector) rated 25A, 2-pole, with a 30mA trip threshold at 230V, in a 2-module (2TE) wide housing. At 25A it's sized for branch circuits feeding a single machine or a small sub-panel; the 30mA sensitivity covers personnel protection (shock) and fire prevention on that branch.
Integration
The 2TE (2-module) width fits standard DIN-rail enclosures — occupies the same slot as a 2-pole MCB. The D=55mm dimension is the depth from the DIN rail mounting surface to the front face; verify clearance against your enclosure depth, especially if busbars or wiring ducts sit behind the device. The Type A (PSE/SSF) marking indicates it meets the selective / short-time-delayed characteristic for coordination with downstream RCDs.