Has there been an implementation of a tag hierarchy with graph edge relations?

1. Can tags be hierarchical?
2. What are hierarchical textual tags?

Can tags be hierarchical?

Tag hierarchies group two or more tag types into a hierarchical structure. The hierarchy defines the order of the tag types, from the broad summary level to the detailed granular level.

What are hierarchical textual tags?

Hierarchical tagging allows you to classify a tag through other tags, exactly like knowledge fragments, and the classification is transitive; that is, if there is a tag “cat” classified with a tag “animal” and you classify some fragments with “cat”, then those fragment will be classified as an “animal” also.