the way I see things:

safe = something you would show to your parents
questionable = something you would show in public
explicit = something you'd get arrested for looking at in your local library.

this should (probably) be put under explicit.

on second thought, Danbooru's lax nature would probably dictate otherwise. oh well.

howto:rate said:

• Questionable
• Exposed areolae, nipples, or breasts.