Part identity and fit
The Siemens 3RH1921-2CA01-ZW98 is a front-side auxiliary switch block with one normally-closed (NC) contact, terminated with spring-type (cage-clamp) terminals. It is designed to snap onto the front of 3RT1 series contactors, adding a feedback or interlock contact without additional wiring space on the rail.
What the spring terminal means on the panel
The 1 NC contact is the normally-closed mirror of the main contactor's state, typically used in safety circuits or as a feedback to a PLC input that the contactor has dropped out. Because it mounts on the front of the 3RT1, it does not consume additional DIN-rail space, which matters when the enclosure fill factor is tight.