The Siemens 3RT6916-1BF00 is a SIRIUS varistor surge suppressor sized for S00 contactors and contractor relays. Rated 400-600V AC, it clips directly onto the contactor — the 9mm depth means it adds almost nothing to the panel depth, so you don't need to re-space the DIN rail or rework the gland plate when adding suppression to an existing S00 build.
Clip-on fastening method means it snaps onto the contactor's coil terminals — no additional wiring or DIN-rail space required. The varistor clamps voltage transients from the inductive coil, protecting the control circuit.