A write-off category is the salvage grade an insurer assigns to a damaged vehicle deemed uneconomical or unsafe to repair. UK categories A, B, S and N describe how badly the car is damaged and whether it may return to the road.
In one line: A write-off category grades a damaged vehicle by severity and whether it can be repaired and driven again.
How a write-off category works
Under the insurer salvage code, Category A means scrap only with nothing reusable, Category B means the body shell must be destroyed though some parts may be salvaged, and both can never legally return to the road.
Category S covers structural damage and Category N covers non-structural damage. Both can be repaired and re-registered if done properly. A car valued at 8,000 GBP with 6,000 GBP of structural damage would typically be declared Category S.
A vehicle's category is recorded against it, so a Category S or N history reduces resale value and must be disclosed to a future buyer and insurer.
Category S vs Category N
Category S vehicles have sustained structural or chassis damage that needs professional repair before they are roadworthy. Category N vehicles have non-structural damage, such as electrical or cosmetic faults, but were still uneconomical to repair.
Both can legally be returned to the road, unlike Categories A and B, but a re-registration and inspection process may apply.
Primary source: GOV.UK: Insurance write-offs