Review tiers are a way of evaluating the relative quality of articles on this site.
Now there are three tiers for article quality.
When an article is first started, it is usually a stub quality meaning it is just a first dot of ink on paper; significant expansion of the index and a definition and discussion of the term graduates it.
When an article has a workable definition, remove the stub template and add it to Category:Defined. This automatically adds the article to the Glossary link on the front page of the site.
The very best articles should be added to Category:Detailed. This will be an informal, and subjective, list of the most expanded articles on the site.