This is a sequence counter UCS Manager increments each time it refreshes the live stats object for this specific fan, independent of the actual speed value reported. It gives a monitoring tool a lightweight way to detect that a new sample has arrived without needing to compare the Speed value itself, which matters since a fan could legitimately report the exact same RPM on consecutive polls. If Update stops incrementing while TimeCollected also stalls, that's a stronger signal of a stats-collection problem than a normal, steady fan speed reading would be.