The Siemens 3RK1903-0AF00 is a 30 mm power bus jumper, designated L123, for the ET 200S distributed I/O system. It bridges the 24 V DC supply rail across adjacent I/O modules, eliminating point-to-point wiring on the backplane.
Snaps directly onto the 35 mm DIN rail alongside ET 200S modules. The 30 mm width matches a single electronic module footprint, so it occupies one slot in the I/O assembly. No additional wiring to the power supply is needed beyond the incoming feed to the first module.