A combination of the words
number and letter, in their literal sense. A lumber can be a letter, a
number, or a combination of both.
Should be used in a conversation where the discussion can be about
letters and numbers, and to avoid mistakes like "What's the answer to number 'b'?", or "Please reference section C... letter 14."
John: "
Hey I'm on question 2 right now. Did you get number 'c'?"
Jane: "Number c? Sorry I have no idea what you are talking about."
John: "Oh, my bad. Did you get lumber 2c?"
Jane: "Yes, yes I did."