Originally Posted by G-BOAC
Both. Once X has run out, I believe you can automatically see V class as award space and it gets converted to X if you book (i.e. it doesn't book in V so you get 25% miles back).
It hasn't always worked that way. The extra award inventory that BA Golds get for BA flights and AA flights in coach has netted me plenty of miles.
I still get award space booked into V that remains as V, at least courtesy of AA.