When describing a girl's hotness to someone, but that girl is either a good friend or your sister (for example), and you do not want to sound like a pervert by talking about them in that way. The number's rating stay the same value, but the negative means that you have no interest in that girl.
Guy 1: I heard your sister single, is she hot?
Guy 2: Dude, that's weird.
Guy 1: Come on, man.
Guy 2: Ok, she's like a -7.
Guy 1: -7?
Guy 2: Yea dude, she's my sister. I'm using the negative scale.