The biggest factors are the color/clarity of the belly and the length. An A belly is wide, white as snow, and has spots. A B belly may be white as snow but now very wide or maybe could look like an A belly but has some stains from milk or other things. The length is a big factor. The cut offs are 40",36",32", and anything smaller than 28" is a kitten and probably shouldn't have been killed. They are very serious about the size. If your cat is 39.99999999999999999 inches they WILL NOT give you the 40" mark. They want to make as much money as they can getting the price as low as they can. So if you have a cat that's close to a cut off do your best to stretch a little extra out of him to insure you get that size. Spots are also helpful especially on the back. If a cat has a plain boring back with no character it will be docked and not be worth as much. The fur length also comes into play but that mostly just depends where your trapping and what time of year it is.