This was posted by our friend Mark Berg at Gofasters:

I will try to clear up what parts fit on what years by giving the insight to the codes within the part numbers.

Generally: the first 2-3 digits are letters to determine what part of what bike.

BT=Chassis Trial
MT=Engine Trial
BE=Chassis enduro
ME=engine enduro
MFS=engine 4stroke
BFS=chassis 4stroke

The next 2 digits are numbers specifying model used. If a part on a 200 shows the "25" number then it is a universal part used on the 200 and 250, in other words all parts that are used on more than one bike is given the generic "25" code.

12= 125
20= 200
25= 250
30=300
40=400 
45=450

The next 1 or 2 digits specify which year first used. With the last 5 digits being the part identifier itself.

ME25"6"44000 means that the part has been used since 1996, even some parts today are from 1996. ME25"03"44000 would mean it has been the same part since 2003. So if you have a 2002 and you want to order something that is a part with an 03 code in it, it more than likely will not fit on your bike. If you have a 2010 model and it shows a part number of ME25644000, it means that part has been the same part since 1996.

Now to through a monkey wrench in this system GasGas has changed the parts numbering in 2008 to have the year and model description at the END of the part number. So "BE915002508" is still a chassis enduro part number that has started being used on a 250 in 2008.