Fulfillment is a category of functions that include Circulation, Resource Sharing, Course Reserves, and Deposits/Move Requests.
Anything that fulfills a request - from a patron or for a library process - comes under Fulfillment.
Circulation Desks are work location(s) in the library that handle patron fulfillment. You would set this as your Location in the Alma menu (upper right corner) so that materials are checked out and in properly.
Fulfillment Units are shelving locations with similar lending/return policies and blocks.
Institution is the campus that contains a library or libraries.
Library is each individual library in a campus.
Item policies can be defined for specific items, and will override any conflicting fulfillment policies for its location.