If you've been involved in a car accident where another party was at fault, especially if the damage was extensive, then your vehicle has suffered diminished value. Diminution in value occurs immediately after the damage to your vehicle has been repaired.
This is because even though their insurance company might repair your vehicle to a high standard, when you go to exchange or sell it on, your vehicle will be not be worth the same value as it would have been had it not been damaged.
Customers will generally not want a vehicle thats been damaged and therefore won't want to pay as much compared to a vehicle that has had no damage history - this is what diminished value is about.
A Diminution report assesses the value of your vehicle based on type and age, extent of any damage, what it will cost to repair and its general desirability. If the accident was your fault then you cannot claim diminution in value.