Digest URI's

I've been thinking of implementing digest URI's but I can't get a good
enough understanding from the mailing list archives to do so, so can someone
help me out please:

1)  Which entitities can have digest URI's

2)  What are they for?

3)  What does a digest URI denote?

4)  What properties do they need to have?

5) I understand there is an algorithm for computing them given an RDF syntax
representation of a model.  Given a model stored in a database, I could
serialise that many different ways.  How do I compute digest URI's for a
model stored in a database, or is that an unnecessary thing to do.
[My assumption here is that the model was constructed in the database, and
was not derived from a serialised RDF input stream]

Brian McBride
HPLabs