The Siemens 3RT6916-1EH00 is a SIRIUS surge suppressor built as a diode combination, rated for 12 to 250 V DC control supply voltage. It clips onto S00-size contactors and contactor relays — the standard SIRIUS footprint for 3RT1 and 3RH1 devices up to about 7 A motor load.
Depth is 9 mm, width 28 mm, height 46 mm. That 9 mm depth is the key number for panel builders: it adds almost nothing to the contactor's panel depth, so you can pack DIN rows tighter than with a separate suppressor block. Clip-on fastening, any mounting position, so it snaps straight onto the contactor coil terminals without extra hardware. Within a standard IP20 control panel that's fine — no derating concern at typical enclosure ambient.