AC voltage detection
My guess is that these are not different windings for 110 V and 230 V; instead, I'd argue that, considering there's a rectifier on the primary side (the large diodes), that this is not just a transformer with a secondary-side rectifier, but, in fact, a switch-mode power supply, controlled by IC1. The second winding is a feedback mechanism for the controller to know how high the energy flow in the transformer currently is, in order to allow for a stable output voltage.