From the GT Manual 6-15:
Performance Points
Each car has a Performance Point rating that indicates its overall capabilities.
In online races, it is possible to restrict entry based on Performance Point ractings. In Performance Point retricted events, the closer they are to the PP limit, the more likely it is that cars with big differences in weight and engine power will be able to compete on the same playing field.
A car's Performance Point rating will change to reflect changes in factors such as max. power, weight, tires and aerodynamics.
A car's Performance Point rating can be checked in the Garage or on the Settings screen.