The Siemens 3RG4012-0JB00 is an M12 inductive proximity switch with a 2 mm flush sensing face, 10-30 VDC supply, and a cable output. This is a standard short-range sensor for detecting ferrous targets in tight mounting spots — think cylinder position, cam detection, or part-present on a transfer plate.
The 2 mm sensing range is the rated operating distance on a standard steel target — derate for other metals or if the target is smaller than the sensing face. Flush mounting means the sensor face can be level with the surrounding metal without false triggering, which is the main reason to pick this over a non-flush variant in a crowded fixture.