<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. For instance, <TAB> was another name for C-i. Later, users found it convenient to distinguish in Emacs between these keys and the “same” control characters typed with the <CTRL> key. Therefore, on most modern terminals, they are no longer the same: <TAB> is different from C-i.
Emacs can distinguish these two kinds of input if the keyboard does.
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.