This read-only field reports the last SONET Automatic Protection Switching (APS) event detected specifically on the working (primary) port of a protected SONET pair on this chassis, using the platform-specific APSEventStatus enumeration. An admin monitoring SONET protection status checks this to see whether the primary transmission path has recently experienced an event (such as a signal failure) that could have triggered or contributed to a protection switch. For example, seeing a signal-failure-type event reported here would explain why traffic subsequently moved to the protect port, as reported in c64SonetAPSProtectPortStatus or c64SonetAPSChannelStatus.