esaProbeHistoryCOS is a value the Class of Service this archived bin's statistics were collected against. On a probe using multiple classes of service, this field is what ties a set of counters back to the specific COS queue they were measured against; on the currently supported single-COS release it simply reports cos-not-applicable. For example, once multi-COS SLA reporting is enabled on a platform, a technician will use this field to separate the voice-COS bin's delay numbers from the best-effort-COS bin's numbers for the very same probe.