Communication is done completely via the typewriter in either octal numbers, decimal numbers, or alphanumeric codes.
It can operate in either the alphanumeric mode or the binary mode.
A decimal digit or a letter (upper or lower case). Typically, "letters" means only English letters (ASCII A-Z plus a-z) but it may also include non-English letters in the Roman alphabet, e.g., e-acute, c-cedilla, the thorn letter, and so on. Perversely, it may also include the underscore character in some contexts.