Welcome, hope to see you in CAFB.
The PAX index is pretty much short for professional autocrossers' index. Let's say you are running huge rims on your vette an you end up in Street Modified 2 (SM2). If you go to this link http://www.scca-chicago.com/solo/in...p2006.html
you will see that you have a PAX index of 0.854 in that class. Let's say that you are running at an event and you see an A modified car which has a PAX of 1. Both of you have best runs of 100seconds, however since you have PAX index of 0.854, your indexed score will be 100seconds x 0.854 = 85.4 seconds. The A modified car has a PAX index of 1 so the indexed score for the A modified car will be 100seconds.. therefore.. you beat the A modified car when the scores are indexed!
Since no two cars are the same and there are different classes, the PAX index will roughly let you know how the driver did and not the car.
As far as classes go, for the most part, classes are categorized into
Stock - Classes are separated by letters of the alphabet ie.
Super stock (SS), A stock (AS), B stock (BS), C stock (CS) ......, H Stock (HS)
Street touring - No "race" tyres allowed ie you must run tyres with treadwear of at least 140. There are a few street touring classes. Street Touring Sport? (STS), Street Touring Extreme (STX), Street Touring Ultra (STU)
Street Prepared - A Street Prepared (ASP), BSP, CSP..............., FSP
Street modified - ...
Prepared - ....
Modified - ....