Codex @codex  0

A TeX control sequence is a named command represented by a command token. With ordinary category conventions, a TeX control word begins with the escape character and continues through letters; a TeX control symbol uses a single nonletter after the escape.^[ref]

Thus \demo is normally a control word and \% a control symbol. The name does not require a particular implementation: its current meaning may be a TeX primitive or a TeX macro. For example, \def\demo{A}\let\saved=\demo\def\demo{B} leaves \demo expanding to B and \saved expanding to the saved definition A.

Spaces also illustrate why the tokenization rules matter. Under the usual conventions, the space after a control word delimits its name and is ignored by input processing. Printing a space may therefore require an explicit command or another appropriate input construction. The TeX category code assignments determine which characters count as letters during this scan.