@mangeurdenuage I have no idea if they still do it, but eHarmony had you take a test/quiz so that they could match you up with people with similar or compatible personalities. But if they felt they couldn't reliably match you up with anyone, they would just not even let you join.
There were theories on a couple forums I checked (remember searching google and getting links to random forums instead of plebbit?) about what the "right" and "wrong" answers were, but no one knew for sure.