This read-only StorageType object reports readOnly(5) for system built-in tone plans and nonVolatile(3) for user-defined tone plans, with built-in plans being added automatically at system startup and unable to be added, deleted, or modified by the user. Administrators use it to quickly distinguish which tone plan entries are fixed, vendor-provided defaults versus which ones they configured themselves and can modify. For example, an admin attempting to edit a tone plan that unexpectedly fails would check this object first to confirm whether it's actually a read-only built-in plan rather than their own custom entry.