Based on what I found, The USA part is the region of the world the game was distributed in for selling. There is also JPN for Japan, EUR for Europe and AUS for Australia, etc.
The LN then stands for the type of game, in this case a cartridge. This could be a demo, game boxes, etc.
Now, the A is stands for the game. It Could also be a Z for a demo
CTR is apparently the 3ds codename Horizon.
Now, the A in AB2E stands for how the game was obtainable, in this case being retail. It could also be J for E-shop things among other things.
The 2B is just a game code. It can be any combination of letters and numbers, but the same for any one game. So every Ultra Moon cartridge will have 2B.
Then E is for English.
Hope this helps!