A320 VDEV in HDG mode.

[Flyman35]

VDEV is available on the PFD even in HDG mode ,provided that crosstrack error (XTK) is less than five nautical miles.FCOM 4.05.6 P07

What does crosstrack error here mean? Which distance exactly they mean?

[Frankie_B]

Distance between the aircraft position (FM POS) and the computed flightplan path.