PAX stands for Professional Autocross indeX. It is also known as RTP which stands for Racers Theoretical Performance.
PAX/RTP factors exists to help provide a comparison between cars competing in different classes. The factors are derived using data from hundreds of autocross events nationwide, including the Tire Rack Solo National Championships, National Tour events, and regional events that take place across the country.
These factors come up often in bench racing, are cited numerous times in the unoffical autocross book of excuses, and have been known to start internet forum exchanges which end in computer keyboards spontaneously combusting.