What this combination starter packs
The Siemens 3RA1125-1EA23-1BP4 is a SIRIUS S0-frame nonreversing combination starter rated for 2.8-4 A with a 48 VDC control voltage. It's a DIN-rail or panel-mount unit that combines a contactor and overload relay in one package — one part number, one wiring pass, one footprint on the rail. The 48 VDC coil suits control schemes running on a DC bus at that voltage. Auxiliary contacts come as one SPDT NO/NC on the MSP (motor starter protector) side and one NO on the contactor side, with the NO contact mounted on the front for easy access. That gives you feedback and status without adding a separate aux block. UL File E156943 in Vol.1 Sec.14 covers it for North American panel builds.
What the 2.8-4 A range means for fit
The 2.8-4 A setting covers motor full-load current in that window. Because it's a combination starter, the short-circuit protection (MSP) and overload are integrated. That saves panel space versus separate breaker + contactor + overload — one DIN S0 footprint instead of three devices ganged together.