It all goes by what VOSA believe to be the first registration date of the vehicle. If the Date of First Registration listed on your V5 document is before 1st January (I think), 1995, then it doesn't require a CAT, as it's only eligible for the idle emmisions test at MOT. Any date afterwards will mean that the car has to have the higher revs emissions test, and therefore will most likely fail without a cat. I may be wrong with some of the specifics like dates, so feel free to correct me anyone! As far as I know, that is the general gist of what it all means.