Cost.

If the amount the insurance company has to pay to fix the car is greater than the market value of your car before the damage, minus the salvage value, it is a write off (totalled).

Say your car is worth 6,000 before the accident, and is now worth 2500 as salvage, this is a net cost to the insurer of 3500 to pay you out. So if the repair costs are 4000, it's a write off. if the repair costs are 3000, they will fix it.

__________________
Cheers, Neil