The Community Edition is a free training platform with mostly java related content.
Users can take exams for which they get knowledgepoints. According to the amount of knowledgepoints they get a belt (ref. judo).
Yellow belt : 0 Knowledge points needed
Orange belt : 18 Knowledge points needed
Green belt : 30 Knowledge points needed
Blue belt : 45 Knowledge points needed
Brown belt : 60 Knowledge points needed
Black belt : 80 Knowledge points needed
Some exams are free. Other exams you can only take in return of some contribution points. These points can be collected by creating and reviewing questions and exams. This way the content is built.
For those wondering: I have a personal account where I got up to the brown belt (black wasn't available at that point) and a corporate account where I'm stuck at some other color. It just doesn't make sense to take those tests again. And the accounts couldn't be merged. Too bad.