Specifies that the Tag's output value is read only. If READ ONLY is selected, users and operators cannot change Value of the Tag in run-time VIEW. Only the field device can update the value of the tag. Alarm Limits and other tag fields are changeable. Typically used with measurements and Input type tags.
If you want to allow some users to change the value of the tag, but not all users, consider assigning a security Area or Level to the tag.