[eschwartz]

Correct, they are synonyms.


sub-groups (hierarchies) are plain tags. The difference is that calibre knows how to split them hierarchically along the "." in order to display them in the tag browser view.
This is an aspect of the tag itself.

In contrast, User Categories are a setting in calibre which groups certain tags or elements in various metadata fields together (it doesn't have to be the "tags" field).



Many (most) metadata fields are NOT tag-like, although they are displayed in the tag browser. Tags are a metadata field that contains a series of comma-delimited values.
"identifiers" does the same.
You can also create a tag-like custom column.