On Book Cave, we use the ebook content rating system from our sister site My Book Ratings (or MBR). MBR ratings are a somewhat like movie ratings, and they're divided into six groups: All Ages, Mild, Mild+, Moderate, Moderate+, and Adult.
Books are assigned ratings based on seven categories: crude humor/language, profanity, drug and alcohol use, kissing, nudity, sex and intimacy, and violence and horror. Within each of these categories there are levels of varying degrees, which are assigned a rating (All Ages, Mild, Mild+, Moderate, Moderate+, Adult, and Adult+). The book itself is assigned an overall rating in relation to the highest level across all categories, and the individual rating level for each of the categories is also listed in the daily deals email and on the book's page.