Its all about the service
The quality of the service relys on the fact that
relationships between the ID and the associated
location(s) of the object (in Internet context, URL)
Relevant questions are:
How to maintain these relationships (mappings)?
How to make updating of mappings between location and
identifier low cost and reliable?
How to guarantee such a service in the long term?
(In Internet based digital library environment PID > URL,
in physical library environment PID > shelf mark)