There are a few catagories for written off cars. It goes along the lines of....
Category A - Vehicle must be crushed. All of it.
Category B - Car may not be returned to road. Parts may be sold.
Category C - Repairable. Possibly structural damage. Cost of damage (at main dealer prices and labour rates) is more than book value of car. VIC check required and V5 will state that the vehicle has been substantially damaged/repaired.
Category D - Repairable. Probably non-structural damage. May have been economic to repair, but insurer doesn't want to.
Category F - Damaged by fire.
Category X - Repairable. Minor Damage.