Assignments are given to a User and govern what a User can see when the User logs into the system. The Assignments are tied into the Organization you have set up within the system.
Let’s say you set up your company with three locations, Location 1, Location 2 and Location 3. If you wanted a User to have access to all Positions inside of a Location 2, you would assign that User to Location 2. When that user logs in to the system, he/she will see all Positions and Applicants within Location 2 (and he/she will not see anything related to Location 1 or Location 3).
You can make Assignments in whatever method best suits your company – as macro as assigning visibility to the entire company to as micro as only giving a User an assignment of a single position.