Full rights is basically like this. Once you give full rights to a person they have the right to re-distribute it and do whatever they want with it. But once you sell for full rights, you cannot sell it again so only 1 person can have full rights. It's the highest ranking of rights along with wearing rights and learning rights. Also when you give full rights to the person, you give the PSD along with it. Snipe.
also if you sell for full rights, i dont think the person you sold it to could claim that they made it...