This is a boolean flag on this specific boot policy stating whether the advanced boot-order model, supporting multiple, granular boot-device paths typically required for UEFI multi-path configurations, applies to it. An admin trying to add a second redundant SAN boot path to this policy and finding the option greyed out checks this flag, since a false value here means the policy is still using the simpler legacy boot-order model that does not support it. A value of true on this policy is what allows it to define multiple SAN or iSCSI boot targets with individual priority, rather than just one boot device per type.