(Note this previous, related discussion: Redundant tags: mixed effects and related models.)
The hierarchical tag is very ambiguous. Unfortunately, this hasn't been addressed before and it now has 142 questions. An excerpt was just suggested:
For questions about hierarchical models, also called multi-level models.
However, it isn't at all clear that that's what the threads grouped under this tag are about. (It isn't clear they are consistently about anything.) For example, at least 33 are about clustering.
I'm wondering what we should do about this. It really seems to be a mess. Some options might be: retagging, making synonyms and/or merging, or deleting.