Imagine you give someone a card with the code "5SBDO0" on it.
In some fonts, the letter "S" is difficult to visually distinguish from the number five, (as with number zero and letter "O").
Reading the code out loud, it might be difficult to distinguish "B" from "D", necessitating saying "B as in boy," "D as in dog," or using a "phonetic alphabet" instead.
We want to write codes that can always be communicated clearly. What's the biggest subset of letters and numbers that will, in most cases, still look visually unambiguous and sound unambiguous when read aloud?