Seen this on the last signal from the Canterbury line at Ashford. It's a double yellow with XC in the box. Seen it before on the western approach from Tonbridge. What does it mean?
By box do you mean the route indicator? The X prefix in a theatre box sometimes (or should that be often?) means the set route is using the particular line (C in this case) in its reverse direction.
At various times and places, depending upon the applicable standards, use of X in this way has been confined to movements towards a Limit of Shunt, so would only then be seen in association with a position-light aspect. This obviously doesn't apply here.