This value reports whether unidirectional link detection is actually operating on a given port right now, which depends on both the local and remote ends of the link supporting the OAM protocol that ULD relies on. An admin checks this to confirm ULD is really active, since simply enabling it administratively via swDULDAdminState doesn't guarantee it's functioning if the far-end device doesn't support the necessary OAM capability. For example, seeing this report disabled(2) even though the admin state is enabled would tell the admin the remote device on that link doesn't support the OAM feature ULD depends on.