What's the connection between the $CI object and cache file?

So I have this piece of code and I've been wondering what's the relationship between the $CI object and the cache file.
This is the code form the Output class, line 449.

PHP Code:

// Does the $CI object exist?
        // If not we know we are dealing with a cache file so we'll
        // simply echo out the data and exit.
        if ( ! isset($CI))
        {
            echo $output;  
             
            log_message('debug', "Final output sent to browser");
            log_message('debug', "Total execution time: ".$elapsed);
            return TRUE;
        } 


What I'm also confused about is how can the $CI object  possibly not be loaded?

Which are those cases?

And as a last thing(as if it wasn't enough), how can the $CI object influence the cache file, or vice versa?


I hope that all my questions are understandable.
Any help will be much appreciated.