Layer 1 - These races are the most common, most basic of the races. Most of these come from the Players Handbook.
Layer 2 - These races are normally variations of the layer 1 races, or the most common of the exotic races.
Layer 3 - These races are the fairly bizarre races, sometimes more powerful, or just relatively rare.
Layer 4 - These races are either extremely rare or extremely powerful, and often quite exotic.