It's a cheap replacement for a distant signal. The coming main signal always shows a restricting aspect or a stop aspect. So you need to acknowledge this with PZB and start braking because you have to slow down or to stop at the next main signal.
There's another one at München Hbf, which you'll encounter if you do a train wash service. Wach auf! it's hiding in the shadow of a bridge.
You'll notice that the same sign is mounted below actual Distant signals. According to the German logic, the sign itself is the Distant signal, and any movable signal associated with it merely allows its meaning to be modified. In some places you'll also see trapezoidal signs. These act as fixed main stop signals. Usually they're only found on the "wrong-side" running line, and are placed to allow bidirectional running in emergencies but with some extra work. You would need to obtain permission to proceed from the signalman. Semicircular signs (usually marked "Halt für Rangierfahren") mark the limit of movement authority granted by shunt signals. Hence you must pay attention to whether you are proceeding under a main aspect or a shunt aspect, a distinction which TSW's HUD does not highlight for you.
