The register is the lip that the inner diameter of the wheel sits on. If you running a hub without a register, the wheel must rest on the spindle itself – limiting how wide you can go.
Hub without a register:
Hub with a register:
I think we usually use Bridgestone wets, but I think that’s mainly because those are the spec rain tire at one of the national series (or were, or maybe my memory sucks). In any case, the B-stones are good rain tires, and in general MG makes great tires so I’d imagine their rains are quality.