The object defines the protected EVC associated on this interface. If the object is uninitializaed (null length string) or is the same value as ostIfEvcName then the object indicates that the protected EVC and the working EVC are the same EVC and their the VIDs are the same. If the object value is different from ostIfEvcName then the protected EVC is different from the working VID. This object is only used (active) if the protected instance is active and indicates the EVC associated with the protected instance on the interface when active.
Sources for this description
Multiple manufacturers; see the original MIB MIB copies (LibreNMS/Observium Community)