I thought it was a good topic as i have recently had a CAT C in part exchange.
CAT C is short for Category C
It is a term used when a car has had an accident and the insurance company dealing with the claim decides it is more cost effective not to repair the car. So the car is classed as a CAT C and the insurance company pay the owner out. However these cars are repairable and can be put back on the road if a body shop or business were to do the work at a cheaper rate. The insurance company could be quoted £4000 for the repair making it not financially viable if the car was valued at say £6000 but the same body shop might be able to do the work them self for £2000 making the car a viable repair.
What is important to check on this type of car is how good the work carried out is and to also have a VIC test done. The VIC test is a vehicle examination carried out by VOSA and they can either pass or fail the car.
So hopefully that is useful information.