This read-only Integer32 identifies which Class of Service a row of ESA probe distribution statistics applies to, for deployments where a single probe tracks SLA performance separately per traffic class. An administrator uses it to correlate a given histogram row with the correct CoS when multiple classes are being monitored under one probe. On Release 4.1 GE206 the description states this always reads 1 because the CoS is explicitly fixed in the probe's own parameters rather than varying per row, unlike a future multi-COS-capable release where an operator might see values 2 or 3 for other classes.