Letter A

When you use 'a', it gives the pretense that you're hypothetically talking about any letter A, B, or c. If you are directly referring to the button SUBMIT below, you don't use 'a'.

Then, when you have a whole slew of options, then saying "the" before every item is tedious, so can you just omit it and directly say "letter A" instead of "the letter A."

You say 'press button B' because there is only one button B amongst other buttons that are of other letters.

If all the buttons are button 'B's, then you'd say 'press a button' and not 'press a button B' since 'B' would become redundant.

BUT, if there are a few button 'B's as well as a couple of 'A's and 'C's, then you can say 'press a button B', better still, 'press any button B.'

Well, in this case, "A" is the item's name. So rather than calling it a knife or a gun, we call it "exhibit A". Sort of a pseudonym. So since A is its name, you call it A instead of "an A". Just like you would say "Look, there is Dyske over there" instead of saying "Look, there is a Dyske over there."

OK, it's exactly the same problem as whether you refer to "my son John" or "my son, John." In the first case you imply that you have more than one son (and this one is named John), and in the second, that you have one son (and his name is John). Here's an example from the AutoCAD drawing I'm working on at the moment

"Enlarge elastomer subassembly view B on the drawing" implies that there is more than one subassembly, but the one the reader should enlarge is a particular subassembly designated "B."

"Enlarge the elastomer subassembly, view B on the drawing" means that somewhere on the drawing there is a view (one picture) of the elastomer subassembly, and the view is differentiated from views of other subassemblies by being given the letter "B."