It means the Reg on the car is not transferrable to another car - you can still put another personalised plate on it at any time.
My Mk1 jag has JAS 901 on it - I have had no end of Asian guys and British Jasons trying to buy the car/plate. This is an age related plate from when it was imported as the plate on the car has to be of the same age as the car, however to stop people importing cheap old cars and effectively being given valuable plates they are always non transferrable.
If I were to put a "private plate2 on the Jag it would have to be a non dating plate (pre 1964 A reg) and JAS 901 would be lost, unless I took the plate back off and then it would be reassigned to that car only
Effectively - if DVLA assign a car a number after inport/plate change it become non transferrable, It doesnt stop you putting another plate on it