User blog comment:B214/High School DxD EX/@comment-31137559-20170415033745/@comment-31137559-20170415053715

When reincarnated devils(only those that are kings I guess) become Ultimate-class, they are able to claim a territory of the Underworld as their own. Those that are promoted to high-class have to make their way on their own. That's why Issei was talking about gathering enough funds to go independent.