I don't know if it's true for all the sites, but on Fanfiction.net the rating system works like this:
K is for content suitable for most ages- 5 years and up, and should be free of any coarse language, violence, and adult themes
K+ is content suitable for more mature childen, 9 years and older, with minor action violence without serious injury. May contain mild coarse language. Should not contain any adult themes.
I post all my stories under this one. Some will have a mild cuss word or two, but I don't tend to do violence much so this works for me.
T is content not suitable for children, but suitable for teens, 13 years and older, with some violence, minor coarse language, and minor suggestive adult themes.
M is for mature teens and older. Not suitable for children or teens below the age of 16 with non-explicit suggestive adult themes, references to some violence, or coarse language. It can contain adult language, themes and suggestions. Detailed descriptions of physical interaction of sexual or violent nature is considered Fiction MA.
MA is content that is only suitable for mature adults. May contain explicit language and adult themes.
I don't think you can post anything MA on Fanfiction.net. I haven't tried but I seem to remember someone telling me that.
I hope this is helpful to you. I'm so glad to see you posting again, your stories are always good!
"You can only be young once. But you can always be immature." —Dave Barry