HEAT LEVELS IN ROMANCE NOVELS are a necessary evil. Authors generally set out to write to a particular heat level, which means they are writing for an intended audience. Sometimes authors write in only one or two heat levels, and others may write books across the heat spectrum.
Authors and publishers assign a heat level to romance books so that readers know what to expect from a book when they are choosing to buy.
Typically, romance books can range from sugary sweet to mind-blowing hot, and anything in between. While there is no consistent "rubric" of heat levels across the romance novel industry, there are some general givens. I've attempted to define them here, pulling information from general knowledge (a 26 year romance writing career) and a cursory review heat levels from publishers and other romance industry professionals.
Level 0. Clean and/or Wholesome. This label originally was designed by readers who were looking to refer these types of books to other readers. A label of "clean or wholesome" generally refers to chaste stories with no kissing, or perhaps one kiss; no talking about or referencing sex; no foul language or curse words; no graphic violence; and of course, no sexual acts.
Level 1. Sweet Romance. There will be kissing, hand holding, touching; very low level of sexual tension between main characters. There is attraction and thinking about the other character. There is no sex on the page, (and probably not alluded to off the page, but maybe.) Always a happily-ever-after ending.
Level 2. Sweeter Romance (sweeter than sexy). Kissing, touching, more kissing, and sexual touching may exist; higher level of sexual tension. There could be one very light, sweet and tender, love scene. Or, there could be closed door sex, or alluding to sex that happened off the page. Story ends with a happily-ever-after.
Level 3. Sensual to Sexy Romance. Kissing and multiple sex scenes happen. The love scenes can be thoroughly and often graphically described. Sex is important to the plot, causes conflict, or defines character -- in other words, not just sex for sex's sake, but sex as part of a developing relationship and leads to the couple's happily-ever-after.
Level 4. Steamy to Erotic Romance. Very sexy and erotic in nature; multiple sex scenes with elements of erotic sex play. Could include multiple partners, light bondage, sex toys, and increasing level of eroticism, along with a developing plot and a happily-ever-after ending (or in some cases, a happy-for-now, satisfying ending)
Level 5. Explicit Erotic Romance, Erotica. Explicit and sometimes extreme erotic romance or erotica; graphic sex scenes described and lengthy. Multiple scenes and perhaps gratuitous sex happens on the page. Stories may contain multiple partners, ménage a trois relationships, elements of domination and submission play, or full-blown BDSM relationships and lifestyle, role play, and subjects that some may consider taboo. Full-blown erotic romance plots may conclude with a happily-ever-after or happy-for-now ending. Erotica may not. Shorter stories may be open-ended. The highest level of sexual heat.
About Women's Fiction and general mainstream fiction novels - These books are generally not rated for heat and graphic levels, but they could be. Books in the mainstream marketplace are often wide open to subject matter and content, including elements of sex, violence, and graphic details of situations and events. Often if a publisher or author is concerned about a graphic situation written into a book--and wants to avoid triggering a reader--there will be a notation in the book description.
Nearly all romance publishers and some independent authors define what heat levels mean for them and they often share them with their readers.
This list is not intended to be all-inclusive, or the final authority on the subject, but a guide for readers and writers. There are many such lists available on the internet for you to search, review, and explore. This is one way to look at heat levels.