ET=Expanded Tankage or Extra Tankage, it's usually an implied designation on 767ERs and 777ERs. For example, the full designation of a United 777-200ER is Boeing 777-222/ER(ET) but people usually drop off the ET
because it's nessecary to have an ER
. All 767ERs are ETs except for some very early 762s delivered to Air Canada and El Al, with only a higher maximum takeoff weight (MTOW) than the standard 762.
SCD=Side Cargo Door. An example designation for this would 747-251F(SCD). On the 747, you don't have to be an SCD to be an F. Remember, the 747 can nose load. For most other planes, SCD and F are mutually inclusive.
B doesn't directly stand for anything. It just means having higher weights on the 747. The 742 was originally the 747B. A few 747-100Bs were built, probably with enough range to handle JFK
-fra with a standard 741. There are no 747-200 base models or "A"s. The B isn't really nessecary, but most publications and official sources leave it on to avoid confusion with the original 747B designation.
X, again doesn't really stand for anything. This should tell you all you need to know: http://airtransportbiz.free.fr/Aircraft/A340-300E.html