I've noticed that some tags such as meaning, meaning-in-context, prepositions, verbs etc., have tag badges. For example, StoneyB has both the 'Meaning' gold badge and the 'Meaning-in-context' gold badge.
However, there are tags that don't have badges such as phonology, phonetics or even vowels!! (at least vowels should have tag badges).
What determines a tag to have tag badges (bronze, silver, gold)? What are the criteria for tag badges?