The Siemens 5SL6303-8CC is a 3-pole miniature circuit breaker (MCB) with a D-curve trip characteristic, rated at 3 A and 400 V with a 6 kA breaking capacity. It belongs to the Basic Devices product family within the 5SL series. That makes it the right choice for motor circuits, transformers, or other inductive loads where the start-up surge would nuisance-trip a B- or C-curve breaker on the same panel.
RoHS compliance is confirmed since June 2013, and REACH candidate-list information is available on request. These cover the substance restrictions needed for CE-marked equipment entering European markets.
The 3 A rating on a D-curve means the thermal element is sized for continuous load at 3 A; the magnetic trip picks up above 30 A instantaneous. That's a wide gap between running current and trip threshold, so verify the cable sizing and the downstream equipment's short-circuit withstand before committing the BOM line.