The pitch diameter is the diameter of the centers of rolling elements when they have the same amount of clearance to both races. The diameters of inner and outer race are calculated by this value, the diameter of rolling element, the contact angle and the clearance.
Usually the mean value of inner and outer diameter of the bearing is taken, but it can be different. For example, it is not valid for a needle bearing without inner ring. The mean value can be selected using the -button behind the input.