The Siemens 3RK1901-3RA00 is an AS-Interface accessory — specifically an M12 addressing cable designed for use with the 3RK1904-2AB00 addressing unit. Its design makes it suitable for K60R addressing, meaning it's the physical link between the handheld addressing unit and an AS-i slave module with a K60R-style M12 connection.
Integration note
The M12 connector on this cable mates directly to the addressing port of compatible AS-i slaves, so it's a field-service item — keep one in the tool kit for commissioning or replacing a slave that needs its address set.