Used in near enough everything.
This is how I see it. (lets use a mk4 ball joint as example)
1J0, 1K0, 1ML is your chassis code. 1J0 - golf 4 , 1K0 - Golf 5, 1ML - Leon I think.
Next part is your Main group:
1 - Engine
2 - Fuel/Exhaust/Cooling
3 - Gearbox
4 - Front Axle/Steering
5 - Rear axle
6 - Wheels/brakes
7 - Pedals
8 - Body
9 - Electrics
0 - Accessories/infotainment
So now you have 1J0 4
next off is the Sub group......2 numbers which refer to a group of parts in the parts manual.
This has a sub-group of 07
So now your up to 1J0 407
next 3 numbers are just random numbers assigned to that part In our case, they are 365 (you will note that a odd number on the end of these set of numbers tells you which side they go on the car......Odd = passenger side)
So we up to 1J0 407 365
next part is the revision. If there have been multiple revisions of a part (N75 for example) VW will superceed the old one with a newer part number.....so that letter on the end then becomes C to a E or E to F or whatever is chosen by VW.
In some cases, such as interior trims, you will see a color code number on the end. B41 is satin black color trim and you will find different codes for different colors.
Hope that helps a bit more.
That is one comprehensive break down!!
