Thanks for the reply, artgeek. That's kind of what I thought. But then, what's the box there for?
This is the first time I've had multiple domains hosted on a single account, and it's only been about 24 hours. But the way the different domains, users, permissions, and other account-level functions and resources are managed through the common Admin panel is a bit confusing at first. Having a box for displaying only domain-specific settings would make sense, but the Active Domain box doesn't seem to do anything like that (or anything at all, as far as I can tell), which kind of adds to that confusion.
A search for 'active domain' in the knowledge base and in this forum didn't turn up anything of immediate use. But this thread about the same issue from last April hat leads me to think the box might not be working correctly now that all DH plans offer unlimited domains.