This enumerated value indicates whether the historical 15-minute bucket referenced by a given interval number actually contains valid stored data, or is notValid because the device hadn't been running long enough to collect it. An admin cares because pulling historical Section-layer counters for an interval marked notValid returns meaningless data, so this flag must be checked before trusting cwSonetSectionIntervalESs/SESs/CVs for that specific past interval. For example, an interval near the 96th slot (24 hours back) on a device that's only been up for 10 hours would read notValid since that far back in history was never actually collected.