CustomProperties which are set as Read-only are properties that values cannot be changed by the user. The read-only properties are used to get their values dynamically based on some specific rule.
Read-only type of CustomProperties cannot be required. When a CustomProperty have been set as Read-Only, the Value is required option could not be selected in the Basic Information page of the CustomProperty Wizard.
The read-only properties can be retrieved from the external data source but they cannot have any functions.