New one on me. Can you advise where you've found this out, out of interest.
B - break for parts only normally
A - Destroy whole.
Only time I can see a CAT B being able to go on the road is if it's written off by a 3rd party's insurer and you've refused to let them have salvage. Can't be sold by a scrappy as far as I'm aware.
its a little agreement between the insurance companys/scrappys that they should be broken, its not a legal thing, therefore if you do happen to get hold of a cat b ( various ways you can end up with one in your posession) then it just needs MOT and vic check and away you go