What it is and where it fits
The Siemens 3RK1903-2AA00 is a TB PEN terminal block purpose-built for the ET 200S distributed I/O system. It provides the PEN (protective earth neutral) connection for a starter, with dedicated terminals for the supply line and load, and mounts on the left side of the starter assembly. The front is finger-safe to IEC 60529, and the protection class on the front is IP20 — meaning it protects against solid objects larger than 12 mm but is not sealed against moisture. This is typical for a dry, enclosed panel environment.