User blog comment:Magma MK-II/Category Cleanup (continued)/@comment-25886777-20160430113525/@comment-366087-20160430163326

It's part of the New and Simplified Villainy Scale.
 * Main—in nearly all appearances/episodes
 * Major—in more than half
 * Minor—less than half, and or henching for a Main or Major
 * One-Shot—One appearance NOT counting mentions, silent background appearances where literally anyone could be in their place, and flashback/clip shows
 * Flashbacks used to present new narrative/content voids this

So no more of primary, secondary, quarternary, true, hidden, etc.

In addition, "one-shot" in particular enables dropping "Recurring", which really is a default-level, common sense category. Any character which is NOT tagged as "One-Shot" is of course recurring, and should be presumed as such.