The Siemens 3RV1917-4A is a 3-phase busbar with an integrated extension connector, designed to link two SIRIUS 3RV2 circuit breakers in Size S00 and Size S0 frames. That 63 A rating tells you the busbar can carry the full motor current of the two breakers it feeds without a bottleneck; if your load sum exceeds 63 A, this bar won't hold.
The plug-in connection means the busbar has no screw terminals on the main circuit — it mates directly with the breaker's busbar ports. In the panel, that saves wiring time and keeps the busbar footprint tight: two breakers side by side, one bar clipped in. No extra terminal blocks needed between the breakers.