What is <ProxyRestriction>?

[Mike Tran <mttran@gmail.com>]

As stated in the core, it's about "limitations that the asserting party imposes on relying parties that wish to subsequently act 

as asserting parties themselves and issue assertions of their own on the basis of the information contained in the original assertion".  What I'm a little vague on is what does it mean exactly for the relying party to create assertions based on the original assertion?  Can someone provide an example use case for this?  Thanks in advance.

-Mike