The “Hard Scale” was created as a better way to rate food. The simple “on a scale from 1 to 10” just leaves too much ambiguity, with almost everything ending up between 6 and 9. More gradations were needed, with more guidelines as to what each number means, so the Hard Scale was born.
The Hard Scale ranges from 10 to negative 10, where zero is indifference. At zero, you literally could take it or leave it. If you are at all hungry and something sits on a plate in front of you uneaten, it's below zero by definition.