It's part of a Polyplumb wet underfloor system. https://wattswater.eu/catalog/regulation-and-control/wired-thermostats/room-thermostat-wfht-dual-with-floor-sensor/
From the diagram below I would assume 2 is live in and 1 is live out. On that basis, for the DT90E, would 2 be connected to A and...