CAT N refers to Insurance group.
Er, I think we knew that. The q was, how is category N defined? We normally see only B,C D and X. In this case I think it means Non-salvage. Other sites seem to use category U as well. A bit of Googling led me to this
http://www.bluecycle.com/about-us/important-information-buying which might be helpful (finding a definitive answer from the ABI site seems hopeless):
“Salvage vehicles in Categories B, C and D are categorised as per the Association of British Insurers Code of Practice for the Disposal of Motor Vehicle Salvage. Categories X and U have been introduced by Bluecycle to assist the buyer when purchasing vehicles outside of this. The categories can be defined as follows:
B: Heavy damage, chassis bent. Not repairable, to be broken for parts and sold to specially registered buyers only
C: Repairable vehicles where the insurer's repair costs exceeded the vehicle's pre-accident value
D: Repairable vehicles where the insurer's repair costs did not exceed the vehicle's pre-accident value
X: Stolen and recovered vehicles with minimal or no damage
U: Vehicles not owned by an insurance company that may have sustained damage.
N: Non salvage vehicles taken in part exchange.”