PAX is a method of handicapping cars. It's also known as RTP (Racers Theoretical Performance).
Cars are assigned handicaps based on their make/model/year and level of modification. These handicaps are based on actual performance of cars that have run in SCCA National events, and are determined by the national PAX administrator. They are used to judge performances at the national level, and are accepted by all competition entrants.
The most modified car is "A Modified", which gets a score of 1.00, so it's time would not be adjusted at all. Everything else gets multiplied by a figure that you could call a PAX handicap.
Here's an example:
Car Class Raw time PAX handicap PAX time PAX Result '03 Corvette SS 50.00 .840 42.0 2 '04 Focus HS 53.25 .780 41.53 1 '04 Dodge SRT4 STU 52.45 .820 43.00 3
So the Focus that finished three seconds slower than the Vette actually wins based on PAX index.
PAX values are adjusted yearly. Click here for the most current PAX factors.