Must have a clear, helpful, and distinctive usage. Tags that can be scraped from somewhere are considered more helpful.
New tags must have a clear, helpful, and distinctive usage for them to be eligible for creation. In other words, if your proposed tag is considered too vague, too niche, or too similar to other tags, it will be rejected by the voters. This of course is very subjective, but nearly everything involving tags will be subjective. New tags will also be considered more helpful if they can be scraped from a studio source already covered on StashDB. So be sure to check for duplicate tags first and make your case to the voters in your edit comment. Adding a category wouldn’t hurt either.
Unconfirmed guideline, subject to change pending formal approval.