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?