It's meant different things over the years - and doesn't mean a whole heck of a lot today. They're just names for cars - S class, E class, C class. E was originally tacked on for fuel injection - einspritzun - or something like that. In other words a 280S was carburated while a 280SE was fuel injected. And then with an extended wheel base it became a 280SEL, for long. It's kind of a hodgepodge of stuff over the years.