A tag group is nothing more than a named collection of tags. The primary benefit of a tag group is that it groups your content dynamically. A document or discussion is not created in a tag group, but will be associated with one or more tag groups if it is associated with tags in the tag groups. If a discussion morphs into something entirely different in time, you can change its tags to change the tag groups it can be found in.
Shows all content, regardless of tags, created in this Community