Pto Horsepower Vs Drawbar Horsepower

What is the difference between a PTO horsepower and a drawbar horsepower rating? I've been looking to buy a tractor, and I've come across both ratings. I'm not sure how to compare them.

In short...if you use PTO you'll have less horse power at draw bar,and even less horse power if hydrostatic transmission.

PTO horsepower is the horsepower available at the PTO shaft.
Drawbar horsepower is a measure of how well the tractor can pull on a calibrated surface.

That being the case comparing drawbar horsepower between tractors would be important for use of draft implements and comparing PTO power would be for useful for PTO-driven implements.

Both should be looked at in comparison to the engine horsepower/fuel consumption for a specific tractor as not all tractors have the same efficiency of transmitting engine power to either the PTO (though most are close) - or to the ground.