a) we want to have a certain type of patron who can borrow books that are normally "not for loan"
One idea might be to create an item type for the books which will be restricted and create a circulation rule which sets the allowable amount of checkouts to 0 for all other patron types. When I tried this and tried to check out the disallowed item to a patron without the proper privilege I got a message, "This patron can't check out this item per library circulation policy." However, it gave me the option to override so that may not be ideal for you.
can i do that by making this a staff patron and then catalog a book as a NOT_LOAN value 2?
You can create custom values for notforloan, but I think they're all treated the same in terms of blocking the item from being circulated. I'm not aware of a way to set a patron category so that you can circulate notforloan items to them. The "staff collection" value is really just an example of how you can create custom labels for notforloan items. The different values > 0 don't have different effects on circulation policy.
b) what exactly are the restrictions in 952$5 and how exactly are they used?
As far as I know nothing in Koha's code uses 952$5 for any kind of policy restriction. As far as Koha is concerned it is informational only. -- Owen -- Web Developer Athens County Public Libraries http://www.myacpl.org