Sub-group in calibre is same as tagging i.e. put a label at a book.
User categorie is a bit like other direction and not only for books i.e. you add books but also authors and more to a created category.
https://www.mobileread.com/forums/sh...d.php?t=118934
There are different but also some similar features to tags and user category.
My recommendation: start with tagging.