What's the difference between Adult and Explicit?

Same here. Also, a detailed explanation should be included in the site's TOS (Terms of Service) or AUP (Acceptable Upload Policy) or something where one can easily find such information on the main site, because I failed to find this information either!

The AUP doesn't exactly state where to draw the line.

All I can think of is that mature is nudity and clothed pinups maybe. And even then there should be clarification if i.e. a boner is still only mature or if it already counts as explicit.

Explicit would be anything that implies sexual acts taking or having taken place.

Mature is tasteful nudity, explicit is porn. I think explicit includes fetish work even if no "technically" sexual acts are commencing.

Correction, I checked the upload policy again, Fetish work is bare minimum mature. For example fully clothed or nude but in bondage is still mature, but add penetration and its explicit.