I'm staying in Hamburg for a while for business reasons and I've been told to get an HVV card for rings A and B as that would cover most of the major destinations I might need to travel to. Now I take it that the bus numbers (e.g., "take the number 2 from Altona") signifies the route (and hence the stops) the bus will cover but what do the rings signify? Is it a collection of bus numbers? A set of routes? A particular loop in the bus network? Something else entirely? I can't find enough info on the HVV website to clarify what a ring might signify.
I'm asking because there are some pubs in Hamburg I'd like to visit and in planning this trip, it'd be great to know if they are reachable from rings A and B too.