With access control, each device will be set up to lead to a specific area.
A practical example, is if you have a general area (that all staff may access), and a server room (that only IT staff may access). There would be a device that opens the general area door, and a device that opens the server room door.
There will be a group (see next section) created for all employees in the IT department, and a general group for the general department
The IT department staff would belong to both the IT department and general groups, giving employees that are linked to both those groups, access to the general are and the server room
While general staff, will only be linked to the general group, which will provide them with access to the general area, but prohibited from accessing the server room