In reply to my own question #92467, what is the term for?..
1. The set of characters in the cipher text is guaranteed to belong to the same set of characters used in the plain text.
It looks as though a ‘finite set’ for the character set and a ‘finite group’ for the characters and the operation is a reasonable place to start. It certainly looks an interesting area to investigate. I hope this comment doesn’t offend and mathematicians who understand the subject. If it isn’t correct, I’d appreciate a pointer in the right direction.