[Koha] Where to set exemption from rental charges

Hi there, I have set up my parametres etc, including circulation rules, but I can't seem to find a place to say which borrower categories are exempt from paying which rental charges (ie by item types). For example, Patrons with category set to "Deaf" do not have to pay rental charges for item types = "DVD" but everyone else does. Thanks for help, cheers Jo.