You wont find rankings at UFC.com. Look on this site or mmaweekly.com. The organizations go by the overall rankings that cover all fighters. Obviously all these ranked guys arent in the same organization. They throw guys into fights when they are new, and unranked, then they get an idea of who is better than who and should be a title contender. If you watch mma for a while you will figure out what tier the fighters are on. Ex: Hughes on top, Diego, BJ, Karo, GSP are top contenders, next are guys like Koscheck, Riggs, Cummo, etc.