I've been keeping an eye on it over the past few weeks due to a few of us being on a different number of credits and they seem to only allow certain blocks on sale at the same time. With the City league game they had to sell tier 1 first, then tier 2, then tier 3. Whilst Tier 1 was on sale, Tiers 2 and 3 were showing as sold out even though they hadn't been put on sale yet. It would be better if they put unavailable rather than sold out as it would be less confusing for the fans.
So with the City Cup game, I assume they are selling tier 3 first, which someone confirmed further up, and tier 2 won't have been put on sale yet. Tier 3 is the bigger of the two tiers and is around 5,500 as far as i'm aware as that is what Hull were given. May be a bit less for us if they are covering the first few rows etc. They will only put tier 2 on sale in the Cup once tier 3 is sold out.
The same has happened with Chelsea away and I think it's because they have 2 allocations of 1,500 and 3,000. The lower tier allocation of the 1,500 allocation has sold out so they are only selling the Upper Tier of that allocation. If we sell that out, they will then put the lower and upper of the further 1,500 on sale.
That's how I understand it anyway.