These are referred to as SIPP codes and are standard throughout the Travel Industry. Each position is a standard category with each letter referencing a specific type. You can do a search for 'SIPP Codes' or 'ACRISS' to find the definition of the different letters... The site I found had a chart but I think it's out-dated and didn't want to post it here.

First letter - size of vehicle (C=compact, F=fullsize,etc)
Second letter - type/or # of doors (things like 'All Terrain', '2 door', etc.
Third letter - Transmission
Fourth letter - Air-conditioning

I think a change was made recently to add additional categories in some of these areas (fuel, etc.)