This option indicates what alphabet is in use, and this affects several things.

• This is the set of characters that will be encrypted. Anything not in this list will be passed through unchanged. Typically this allows spaces, punctuation, etc. to show up in the encrypted text in the same way that it shows up in the input text.
• The keys need to be the same length as the alphabet. Right now, you cannot select the key, so this is taken care of for you.