csOc3AtmIntervalUnCorrectedHEC is a read-only 64-bit counter of the number of ATM cells with an uncorrectable, discarded Header Error Control error, recorded for one specific past 15-minute interval bucket on this OC3 port. Because uncorrected HEC errors mean real cell loss, having this per-bucket lets an admin identify precisely which 15-minute window actual data loss occurred in, important when reconciling a customer's reported service disruption against the exact time it happened. For example, if a customer reports a brief ATM service disruption at a specific time, an admin can check the matching csOc3AtmIntervalUnCorrectedHEC bucket to confirm real cell loss occurred in that exact window rather than the issue being elsewhere in the customer's own network.