They are generally chassis codes.Â* If you venture over to Wikipedia.org, you will generally see multi-generation vehicles broken out into their respective chassis codes.Â* From the BMW 3 Series page (since you brought it up): Contents [*]
1 Generations[*]
2 E21, First generation, 1975â€"1983[*]
3 E30, Second generation, 1984â€"1992[*]
4 E36, Third generation, 1990â€"2000[*]
5 E46, Fourth generation, 1998-2006[*]
6 E90, Fifth generation, 2005â€"2012[*]
7 F30, Sixth generation, 2012â€"[/list]etc. ps. E39 is a 5-series, Philistine!Â* :-)