Standard Length Messages over RS-485
There is no "variable A" - there is only a macro called "A" which gets replaced by the contents of the #define for "A" at compile time.

You have to remember that the "A" that the Arduino receives is just a number.  It is only us humans who choose to associate that number with the letter A, and write software that takes that number and display it as an A.  The letter A is number 65.  The mapping between numbers and letters is called ASCII - or American Standard Code for Information Interchange.

What you do with that number is entirely up to you.  If you want to use it to display pre-defined messages, then there's a number of ways you could do it.  You could:


Use a series of if statements to compare the number you have
Use a switch/case construct in a similar way
Store the messages in an array and use the received number as the index


For string constants the last option is tricky to get right on an Arduino where there is very limited RAM - you have to ensure that you use PROGMEM and access it properly, and doing that from an array is not as trivial as it would otherwise be.

The best method would probably be the switch/case option:

switch (letter) {
    case 'A': Serial.println(F("First Message")); break;
    case 'B': Serial.println(F("Second Message")); break;
}