These are the ratings for stories - Safe, Moderate, Mature, and Extreme.
A story rated safe is completely safe for anyone of any age to read. It contains little to no violence, little to no sexual content, and little to no profanity.
A story rated moderate is your average MI story - moderate violence, moderate sexual content, and moderate profanity.
A story rated mature has an excessive amount of one or more of the defining rating tools. It has either a large amount of violence, sexual content, or profanity, or a combination of both. Readers must be mature enough to read it. (i.e., 13+, 15+)
A story rated extreme has an extreme amount of violence, sexual content, and profanity.