The key names used by send-keys are the same ones that bind-key uses.
From the Key Bindings section of the tmux manpage:
When specifying keys, most represent themselves (for example ‘A’ to
‘Z’). Ctrl keys may be prefixed with ‘C-’ or ‘^’, and Alt (meta) with
‘M-’. In addition, the following special key names are accepted: Up,
Down, Left, Right, BSpace, BTab, DC (Delete), End, Enter, Escape, F1 to
F20, Home, IC (Insert), NPage/PageDown/PgDn, PPage/PageUp/PgUp, Space,
and Tab.
Although they are not listed in the man page, there are also special names for keypad-specific keys: KP0 through KP9, KP/, KP*, KP-, KP+, KP., and KPEnter.
Several of the more cryptic key names (BTab, IC, DC, NPage, PPage) probably come from the terminfo library.
Emacs shares the convention of using C- and M- prefixes to indicate modifiers (I would not be surprised if there were earlier uses of this convention).