These requirements are in my opinion out of touch with operational
reality; networks evolve and change over time, and given enough time,
pretty much all assignments made will end up being used in a different
way than what the original criteria was. The PI assignment request form
asked for an very detailed criteria, with a breakdown of each individual
subnet in the network, and a listing of all the equipment that would be
used, including the manufacturer name and model numbers. So if you in
the 1990s received an assignment that you had said was for a dozen brand
new Sun UltraServers, you better not have replaced those with modern x86
hardware, or you have invalidated your assignment! :-O

Some NCC staffers have told me that the way they've logically "solved"
this impossible requirement was to consider that whenever the
criteria/purpose changed, the original assignment was returned, and a
new one consisting of the same block was issued for the new purpose.
Then they could just "optimise out" the middle steps where the
assignment was removed and re-added. That approached worked (up until
Sep 2012 anyway), but I think it would be much better if the policy
didn't worry so much about the "original criteria", but rather focus on
whether or not the assignment conforms to the address policy in effect
at any given time. If it does, there is no reason to call it invalid.