those are just guidelines used in the salvage and insurance industrys,
a salvage yard is not allowed to sell a cat b vehicle complete, but a private person in possesion of a cat b vehicle can repair it, take it to a vic test and get it reclassified to cat c, all flood damage cars for example are automatically cat b but the majority are repaired, reclassified and resold, i myself have handled some of these and had them put back on the road, the vic test is a lot more involved than one for a classc or d vehicle
im pretty sure james at motorsport developments used to vic test class b's and will confirm what i am saying. or you could just contact your local vic centre and they will tell you all you need to know