They refer to the different chassis models. In other words, with each body style change of each series, the chassis model changed.
From what you listed, the E32 is the 88-94 7-series, E34 is the 5-series of the similar year, E36 is the 3-series from 90-96 I THINK, E38 is the 95-02 7-series.
Wikipedia "BMW Chassis designations" and you'll get a more extensive list.