Explanation
Devices Enrollment Modes
Open Enrollment:Essentially allows anyone meeting the other enrollment criteria (authentication mode, restrictions, and so on) to enroll. Workspace ONE Direct Enrollment supports open enrollment.
Registered Devices Only:Only allowed users to enroll using devices you or they have registered. Device registration is the process of adding corporate devices to the UEM console before they are enrolled.
https://docs.vmware.com/en/VMware-Workspace-ONE-UEM/9.5/vmware-airwatch-guides-95/GUID-AW