Ed,
Those definitions were given to me by a noted porting shop that does Flowbench for for many local race shops.
Stage I = blueprinting of ports, removal of casting flash, smoothing of transitions, multi-angle valve job with results slightly better than stock.
Stage II = enlarge ports while maintaining smooth airflow. Significant increase to flow volumes, but maintaining high velocity during mid to maximum valve lifts. The objective is to increase torque at all ranges and reduce the restrictions up to the maximum usable rpm limits of the surrounding components.
Stage III = maximize airflow at high valve lift/openings. Increase and optimize the amount of air able to flow into a filling cylinder at high rpm. While low rpm and some mid-range velocity and cylinder filling is lost at mid-range rpms, the stage III porting allows maximum filling of cylinders at high rpm for torque peaks high up in the rpm range for higher, maximum horsepower.
Regards,
Gordon