I want to treat myself to a Cat 1 seat opposite the Club Wembley seats but does it mean that until all other blocks are sold out this particular area won't be available, in fact it would be the last.
I would have think you choose what category you would like first - thats the way that would make most sense to me. Otherwise you would have people potentially buying cheaper tickets than they might have actually wanted etc
The Cat 1 seats currently on sale are in the lower tier, near the halfway line. If you want Club Wembley block, the only option is currently Cat 3 behind the goal, which is what I've gone for.