A typical xMatters instance will pull items from a data source, such as LDAP or Active Directory, to populate users and their devices, among other things. All of the various aspects of a device can be set according to values coming over from the data load, such as Active, Default Device, Country, Priority Threshold, etc.
However, often times, some of the pieces coming over should not be editable by the end user, while others should be. A good way to get around this is to set the EXTERNALLY_OWNED flag on the object in question. Then, navigate to the Admin > Externally Owned section, which allows an admin to lock down individual data elements.
For example, the Email Devices have these entries:
This setting locks down the Priority Threshold which disallows the end user from altering the value:
Note that the priority threshold is literally locked down, but the rest of the details are editable.