@ Xavier:

disclaimer: this is not to criticize but (hopefully) to provide constructive
input

thank you for your efforts in trying to explain this setting. However it's not
clear at all for me. For example, what's the difference between "absolute URL"
and "original URL"?
Also: you don't seem to use correct terminology, which further complicates the
issue. As per www.en.wikipedia.org/wiki/Uniform_Resource_Identifier and -
particularly -
www.en.wikipedia.org/wiki/File:URI_Euler_Diagram_no_lone_URIs.svg a URL *IS* a
URI. The one is acutally a subset/subcategory of the other. Maybe you mean
URN, as opposed to URL ?
Regardless of terminology, the best way to explain it without ambiguity to
users is to take a concrete example, e.g.

<http://i1.nyt.com/images/misc/nytlogo379x64.gif>

and specify:

relative URI = ...
absolute URL = ...
absolute URI = ...
original URL = ...

(I used your terminology here).