From the UWZ perspective, then...
Wait is, simply, a reserved AC that may be used at any time - it does not, necessarily, have to be in direct response to the AC of an enemy model.
All models have 360° LOS, therefore, the model can use the AC (assuming that it has successfully passed an LD test) to perform a non-movement AC. Attack AC cannot occur if the model would have to move, first, to be able to bring the target into the model's Fiing Arc (except Charge/Counter-charge, as these AC's are Attacks with the benefit of Movement).
If the model that is on Wait is the target of a Charge, there is no need to roll a LD test, at all, to use the AC to Counter-charge, to Brace for Charge or to Withdraw.