What is the xo connection in a transformer for?

The X terminal identification on a transformer designates the secondary side of the transformer. Depending on the type of secondary windings, the terminals can be designated as X1, X2, X3 and X4. On a single winding secondary the terminals will be X1 and X2. The "hot" wire will be supplied from the X1 terminal and the return wire will use the X2 terminal. On most control Transformers the X2 terminal should be grounded to the control panel's chassis.