Three Simple Rules

The sexual tension is the definition of demure. There's some kissing, but love scenes (if any) are behind closed doors or fade to black.

There's heavy petting and at least two intimate scenes with descriptions, but it stays on the tamer side. There's sugar and spice, but it sticks closer to the nice.

The heat level gets cranked up a notch with all the passion and lust mixed in. We're talking two to three intimate scenes, with two steamy scenes being explicit with descriptions and euphemisms.

The spice detector is going off here. Books with four flames have at least three explicitly detailed sex scenes. These books are on the naughty side, but the steaminess doesn't overshadow the plot.

The spiciest book you can order on the spice menu. These books are erotica and focus primarily on sexual activity. The sex scenes are highly explicit and can last for pages.