Due to a recent query from a forum member I have decided to list here the full description of all the categories write-offs as applied by UK insurers, it should help you understand what these terms means when attempting to buy a second hand bike that has been written off by an insurance company. Be careful that when it says repairable, it doesn't always mean it can be re-insured as some insurance companies will not even consider it. So check with them first before buying.

• Category A - Vehicle must be crushed. All of it.
• Category B - Bike 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 bike.
• 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.

In more detail:

Category A - May not be resold. Entire motorcycle must be crushed. Total burnout or flood damage (salt or foul water). Severely damaged with no serviceable parts, or already stripped out shell. DVLA require "Notification of Destruction".
Category B - Motorcycle itself may not be resold. Damaged beyond economical repair, usually with severe structural/frame damage. DVLA require "Notification of Destruction". Parts can be removed and sold.
Category C - Repairable salvage. Usually applies to motorcycles with significant frame damage, where cost of repairs exceeds book value. Can be sold complete to Motorcycle Trade or Public. Recorded as "Category C" at DVLA. Category C vehicles' V5 documents are returned to DVLA. You re-apply, to DVLA or at your local VRO, for registration on the original identity once you have fixed it up, MOTed it and want to Tax it. Re-registration removes the Category C classification, but evidence it was at one time Category C remains on the vehicle's record at DVLA (and HPI and AA and the others).
There is a difference between cars (and presumably vans, lorries, caravanettes..) and bikes when re-registering. Motorcycles do not need a VIC inspection, cars do. Cars (and vans...) sold for repair but must now have VIC inspection before returning to the road.
VIC inspection - Straight from VOSA's web site "[The VIC] will involve comparing the vehicle presented against information held by DVLA, such as the vehicle identification number, make, model, colour and engine number. The VIC will also compare the record of previous accident damage with evidence of damage repair as well as checking other components to confirm the age and identity of the vehicle."
Officially, the VIC does not check roadworthiness. If any significant defects are present, they can prevent it being used, but it is not a check of vehicle condition or roadworthiness. VIC tets centres in major towns, list on VOSA's site. It costs �35 (early 2005).
Motorcycles do not need this VIC inspection. (Lets not get into whether this is a good or bad thing - you probably have a view somewhere between 'one less hoop to jump through' and 'so do they not care about stolen bikes being rung !'). Whatever, bikes do not need a VIC inspection.
It used to be unclear, but VOSA have recently (late 2005) re-written some of the pages on their web site. Some of their pages still say ' all vehicles', but some pages now say ' cars need....'. From 2 sources, we now have clear evidence of people being told, one in writing, that "Motorcycles do not come under the Vehicle Identity Check Scheme therefore your vehicle will not require one.".
Having said that, when you go to insure it, the Insurance Company will obviously know it was Category C, and may insist on an Engineer's Report on the quality of repairs and the roadworthiness of the vehicle. Some do, some don't.
Category D - Repairable salvage. Minimal damage, probably not structural, but insurer does not want to repair, even though it might be economic to do so. Often stolen and recovered after claim has been paid. Or for unusual models or grey imports where the difficulty of obtaining new parts hinders a quick repair. Does not need VIC inspection to return to road. Recorded with HPI, AA, and the like.
Category Theft - Reported stolen, and the insurance company has paid out, but nobody has found the bike yet. (Once it is found and taken to a salvage yard, it probably becomes Category D(or ABC...))
Category F - Damaged by fire. Should be repairable. If not safely repairable, it should be called Category A or B.
Category X - Insurance-speak for 'Not Recorded', but subject to some claim and sold on by them as repairable salvage. Not recorded on registers at HPI, AA, .. or known at DVLA. Very light damage, or vehicle is fairly new. Requires minimal repair work.
"not recorded" - Not an official category, this is what you may see in a private advert. Usually means the owner only had third party insurance and they stuffed it themselves, so can't claim for it. Naturally, they aren't telling their insurance company - and they haven't told DVLA, so it is not Recorded with them, or HPI et al. Might be a total wreck, might be fixable. Buying something like this, you are on your own, and you'd better know what you are doing mechanically.