This read-write field records current ownership of a Security Sensitive Data rule, and per the description, an admin can flip a system-default rule to user ownership (and back) whenever rlSecSdRuleIsDefault is true. Admins care because converting ownership is typically the required first step before customizing a built-in security rule. For example, an admin flips a default rule's ownership to user so they can then edit its access level, something a locked default rule wouldn't otherwise permit.