The Siemens 3RK1903-0AF20 is a bag of 20 caps for the power bus on ET 200S motor starters — the plastic covers that snap over the exposed busbar ends on a populated motor-starter rack. If you are maintaining or commissioning an ET 200S line, these caps are the standard way to close off the power-bus segments between starters; without them the busbar stubs are exposed at the end of the rail.
The substance prohibitance date of 05/01/2012 indicates the RoHS compliance cutoff for this part number — material composition meets the EU RoHS directive as of that date.