Category D is a level of damage insurance companies use to describe vehicles they have written off. After October 2017, they were known as Category N vehicles. Learn more about Category N vehicles here.
A Cat D vehicle will have suffered damage in the past, probably in an accident. The insurance company that handled the claim decided that repairing the vehicle would have cost more than replacing it.
Insurers often sell Cat D vehicles for salvage. Many are safely repaired and returned to the road.