I don't have subcollections; I have a deck for each aspect of my life or a perspective which favours the subject of my inquiry.
When I used to be a collector, I grouped by historical and religious themes. But over time I naturally I found myself working with some decks more than others. Practically, rather than leave the others on the shelf or in the closet to gather dust, I let them go. If they weren't going to be used, I didn't want to own them anymore. Most of them now have better homes with friends who use them, and the ones left are ragged and faded with love and use.
However, my remaining decks are definitely divided by the spiritual and the mundane. The spiritual ones I use for guidance with exceptionally painful, burning questions about how I should view the world and whether I'm really following my heart. The mundane decks are guidance for how I interact with others and how my personal philosophies and views are channelled to practical ends.
Naturally there is a lot of overlap, and sometimes I'll just grab one at random because I'm in the mood for it. But I try to value each for what it gives.