This is the companion unit for the 5SL4 line, not a standalone device; you bolt it above the MCB on the rail, and the top-side supply feeds through the RC unit first.
The 300 mA trip threshold is the key spec for this part — that's not a personnel-protection level (those are 10 or 30 mA); it's an equipment-protection / fire-prevention level, typically used on circuits where nuisance tripping from leakage in long cable runs or industrial gear would take down a 30 mA RCD. The 3 kA surge current resistance means it'll survive lightning-induced or switching transients up to that peak without damage.
Mounting is standard DIN rail (EN 60715) — three modular width units wide (54 mm), 70 mm deep, 100 mm tall. The position on the rail is top-side, above the miniature circuit breaker, with supply coming in from above. Any mounting position works. The 10 000 mechanical switching cycles typical service life is fine for a distribution-board device that sees infrequent manual operation or fault clearing; it's not a contactor-duty part.