Originally that was for the water heater. I could never figure out WHY they labeled it as LP heater though. The engineer (that is no longer here) said, "well the flame is in the shape of water"....of course that also happens to be the same shape as a flame...
But the XL would have the Aquahot, not the Truma water heater...but in standardizing the multiplex...maybe that icon slipped in and activates a blank circuit.