This flag tells you whether the current-day error/second counters (ESs, SESs, CVs, FCs, UASs) collected so far today are trustworthy, or whether something interrupted the measurement. It matters because a low error count is meaningless if the bucket is flagged notValid -- for example after a card reload, reinsertion, or clock loss partway through the day, which would otherwise make a broken path look artificially healthy. If a technician swaps the line card at 2pm and then queries this flag, they should expect to see notValid(1) until a full day's worth of clean measurement has accumulated again.