The Siemens 3RK1903-3AE00 is a TM-PFX30 S47-G1 potential group module from the SIRIUS family, designed for the beginning of a new group without infeed from the left.
This module is used in motor control centers and distribution panels to create a separate potential group — isolating the power supply for a new section of the load feeder. The 30 mm width keeps the panel density tight; it occupies one standard DIN-rail slot.