The Siemens 3RT5955-5AB31 is a SIRIUS magnet coil, the replacement coil for a size S6 power contactor. It's the part that pulls the main contacts in — no coil, no switching. Coil voltage is 26 V AC/DC, so it accepts either supply without a separate rectifier or tap change.
What the coil rating means for your panel
That means the same coil works on a 50 Hz line in Europe and a 60 Hz line in North America without swapping parts. The AC/DC designation also simplifies the control transformer selection — no need to segregate AC and DC control buses.