i 'think' :

cat A is to be crushed

cat b is breakers (only to be bought by people with breakers licence)

cat c requires VIC check before going on road

cat d doesnt need a VIC check but has been wrote off (usually stolen with/without keys and paid out before it was found, or minor damage on worthless car)

if it is your car i believe you can contest to have the category be lowered. no matter what category, the car will just come up as 'write off' on a record check (HPI, RAC etc)

hope that helps, and correct me if i am wrong