PAX stands for Professional Autocross Index and is a handicapping system that is used to compare the performance of potentially very different vehicles.  Stock vehicles because of their basically un-modified state have the lowest PAX factor and Modified have the highest (or no) PAX factor.  Each vehicle's PAX factor is multiplied by the raw time in seconds for each lap around the circuit yielding the PAX time.  It is analogous to a golf handicap and works like this:

The HS driver betters the AM driver's PAX time event though the raw time was more than 16 seconds slower.

For an updated list of actual PAX indexes check out the entry in our FAQ entitled "2009 PAX Indexes"