The letter designations USED to have meanings. But no more. They're just letters designating the level of standard equipment. SE has a lot more standard bells and whistles than a DX, that's all.
Look at the Honda Ridgeline for example. RT, RTS, RTL? The letters have absolutely no meaning except to denote the level of standard equipment.

Wow, thanks for the answers I got so far, still curious if anyone knows what the abbreviations originally stood for, or if it was just an A,B,C sequence thing. At least now I have an idea what they designate. (For a "Girl" that was raised always helping my step-dad fix our cars, I've always been one to actually want to actually "understand" the different parts and what they mean.) Here's to being continuously curious and always increasing knowledge.

Actually, the letters never really meant anything. With the possible exception of the 1961 Impala SS which never had an official designation of "Super Sport". "SS" just sounded cool. And Chevy just let customer imaginations run wild as long as it sold cars.
In 1964 I purchased new a Ford Galaxie 500XL. I know for a fact XL NEVER meant "Extra Luxury" as some claimed.
The auto companies do focus groups to ask what sounds best and most appealing. They throw out these designations and the group either goes "aaaahhhhh" or "eeeeccccchhhh". And the "aaaahhhhhs" have it.
So don't go crazy trying to figure out LX, GT, MYW, LSMFT or any other nomenclature.
With the exception of GTO, which was copied (stolen) from Ferrarri, they mean nothing.

442 meant something. At least originally.

4 speed
4 barrel
2 exhaust

GTO stood for "Gran Turismo Omologata", Italian for "Grand Touring Homologated."

"GT" ("Grand Touring") used to have a fairly-solid definition, too. Now it simply means fancy wheels and pinstripes -- at least for most manufacturers.

The ancestor of Mazda's current 3 series was known in the U.S. as the "GLC", which Mazda insisted stood for "Great Little Car".

And sometimes the numbers follow a pattern, like Peugeot's use of n0n (405, 504, 604, etc.) and Mazda's use of n2n for a while (121, 323, 626, 929).

Volvo used to have some nomenclature also. 240 series meant 2 series. 4 meant 4 cyl. and 0 was broken down to the number of doors. 242 (2 door) 244 (4 door) and 245 (wagon).
Yes, in 1965 442 DID mean 4 bbl, 4 speed, dual exhaust. But that too got watered down when more cars were sold with automatics. And Pontiac swiped the GTO from the Ferrarri GTO. But we all knew what those letters REALLY meant when applied to the "Goat". GTO... "GET the TOOLS OUT!"

i took out a loan on a ford ltd in my younger days, think the ltd stood for long term debt hahahahaha

Ok so with this, can anyone tell me how I would find out if my car is a LX or DX or SE? (2000 Honda Civic sedan)

I pulled out my registration and looked on it. It clearly stated 2012 honda civic dx