Don't know about the "background", but Wizz has:
3 seperate AOC's. MAin one is Wizzair Hungary. Then there is Wizzair Bulgaria and Wizzair Ukraine.
The seperate AOC's for Bulgaria and Ukraine is, I believe, due to the fact that these countries are (or were) outside the EU when operations started there. Hence, different AOC's were needed to operate and bid for routes.
The Hungary AOC covers all bases and aircraft that do not operate under the Bulgarian and Ukrainian AOC's. This is the vast majority of the current fleet. I believe WAB and WAU only have 2 aircraft each currently.
Don't know about contracts. I try and keep it simple, so am only interested in what I am paid.
Hope this helps.