I would be grateful if some one in the house can tell me the meaning of the numbers on vehices such as PGT 306, PGT 406, AUDI A4, MERCEDES C200 etc.

It depends on the automaker. Some automakers number their vehicles according to vehicle class (SUV, compact, etc), by order in which the car was released, or some by the number of cylinders in the engine. The general system followed by most automakers consist of one or more letters, usually indicating the vehicle class. For example, Mercedes: S for the large saloon, E for the mid size saloon, C for the small saloon, G for the offroad truck, etc). The letter(s) would usually be followed by a series of numbers, usually indicating the car's engine displacement. I will use Mercedes as an example again, The C200 would indicate their small saloon with an engine displacement of 2.0 Litres. An S430 would be their large saloon with an engine displacement of 4.3 Litres. I hope that this is helpful.

thank orimili , that mean i have to do more research.