What's the difference between Cat S, Cat N, Cat C and Cat D?

All four are insurance write-off categories, but they come from two different systems. Cat C ("total loss, repairable") and Cat D ("total loss, repairable, lower value") were used until October 2017, when the insurance industry replaced them with Cat S (Structural) and Cat N (Non-structural), to focus on whether the damage affects the car's structural safety rather than just the cost of repair.

The old and new systems measure different things: Cat C and Cat D were mainly about repair cost compared with the car's value, while Cat S and Cat N are about whether the car's structure was damaged. So a Cat C car isn't automatically the same as a Cat S one, and a Cat D isn't automatically a Cat N. A car written off before October 2017 will still show its old category, which isn't an error.

Whichever label a car carries, the practical points are the same: it's had a more serious history than a car with no write-off record, the quality of any repair matters far more than the category itself, and it's likely to affect the car's value.

Source: gov.uk: insurance write-offs