To answer your question on why PDM generates : "pdm location x.x.x.x x.x.x.x, here is an explanation from Cisco TAC:
A PDM location is a pure book keeping command used by PDM to build its topology database.
It has nothing to do with the PIX's functionalities. In particular, it does
**NOT** control which host can access PDM which is a common
misunderstanding.
The control is done by the command "http ".
Why do we need it?
In PDM's world, policy (those rules) is built on top of topology.
Ideally user creates the topology first via the Host/Network tab, then
configures policy else where (like Access Rule tab). A network object
exists by itself, even if there is no policy configured directly on it
at a particular time. We use "pdm location" command to remember the
location of a network object.
I hope this helps and please rate this post if it answers your question as others might be looking for the same answer on PDM locations.
Jay