Basically, everyone who scores us falls on the 0-10 scale.
If a partner rates us 0-6, the partner is a detractor and they are unhappy with our service.
If a partner rates us 7-8, they are a passive and they neither promote or criticise us.
If a partner rates us 9-10, they are a promoter and will like what we do and recommend us to others.
What we then do is to subtract the % of people who are Detractors from the % of people who are Promoters. This gives us a score of anywhere between -100 (this will be terrible- as it means everyone is a detractor) and 100 (this is awesome- as it means everyone is a promoter).
Its a pretty brutal grading to be honest, particularly as people tend to gravitate around 5-6 if they have neutral views about the service we offer. What it does do, really well, is provide a useful holistic view as to how we are matching our service against the expectations of our customers.