About <DEL> character and Control character.

[waterloo]

I am reading Emacs Lisp Reference and can not understand the following paragraph:

For historical reasons, Emacs treats the <DEL> character as the
control equivalent of `?':

     ?\^? => 127     ?\C-? => 127

As a result, it is currently not possible to represent the character
`Control-?', which is a meaningful input character under X, using
`\C-'.  It is not easy to change this, as various Lisp files refer to
<DEL> in this way.

What I understand is :  
?\^? = <DEL>
now we can not represent `Control-?' with `?\ " syntax.

Is it right?

What does "which is a meaningful input character under X, using

`\C-'. " mean ?

Thanks