TAB, RET, BS, LFD, ESC and DEL started out as names for certain ASCII control characters, used so often that they have special keys of their own. Later, users found it convenient to distinguish in Emacs between these keys and the "same" control characters typed with the CTRL key.
Emacs 19 distinguishes these two kinds of input, when used with the X
Window System. It treats the "special" keys as function keys named
delete. These function keys translate
automatically into the corresponding ASCII characters if they
have no bindings of their own. As a result, neither users nor Lisp
programs need to pay attention to the distinction unless they care to.
If you do not want to distinguish between (for example) TAB and
C-i, make just one binding, for the ASCII character TAB
(octal code 011). If you do want to distinguish, make one binding for
this ASCII character, and another for the "function key"
With an ordinary ASCII terminal, there is no way to distinguish between TAB and C-i (and likewise for other such pairs), because the terminal sends the same character in both cases.