This object indicates whether the lockout administrative command has been applied to this specific input clock source, which per the description means the selection process will not consider it at all, regardless of how good its quality level might be. It matters because a locked-out source is effectively invisible to automatic failover; if an admin forgets a lockout was applied during earlier maintenance, they might be confused about why a perfectly healthy, high-quality source is never selected. Seeing true here on what should be the primary GPS reference would explain an otherwise mysterious situation where the device keeps running on a lower-quality secondary source instead.