A given channel lives in a single category (much like a topic does), and there can be more than one channel in a category.
So, for example, we could have channels for #chat and #admin-ui both live in the #feature category here on meta, which would be accessible to all logged in users.
We could also have separate channels in the lounge category for TL3 members named something like #lounge for casual conversation and #content-gardening for discussions about improving topic titles and tagging/categorizing existing content better among TL3 users with those permissions.
Channels can live in subcategories as well, but there isn’t really a concept of a single channel being associated with multiple categories, so nothing like “enable for all subcategories.”
Does that help? If not, can you share a bit more about what you’re thinking you want to accomplish with how you set up chat channels?