The first letter is determined by the type of license the car is under:
Private cars are assigned - P, hired cars (maxis, taxis, buses, etc.) - H, transport vehicles (trucks, vans) - T, etc.
The other two numbers just run along the alphabet from P to PA to PB and now to PC.
At this point in 2010, the PCR series is out and the molecular biologist in me smiles every time I see it.