Seems the admins finally realized that people might want to limit NSFW use of their models without the model being assumed to be intended to generate minor characters. For those unaware, that middle checkbox didn't exist as little as 9 days ago.
Edit, the options do have different effects.
"Minor characters" locks generation to PG.
"Not for NSFW" only locks it out of x and xxx. (I guess R is SFW now ¯\_(ツ)_/¯.)