That is interesting I didn't realise the FAA did not use it. 'Damp' is always a hard one to call. especially on a grooved surface. I always check the bottom of the grooves as the water is sometimes retained there even when the surface is dry.
Pilots have told me before that even if the runway is called 'Damp' then they treat it as 'Wet'. That would tie in with the FAA rulings I suppose.