This reports the lowest Eb/No (energy-per-bit to noise ratio) the demodulator saw during the sample period, in tenths of a dB, with 999 flagging that the demod was unlocked at some point. Because it is the minimum rather than the average, this value reveals brief fades or rain-fade dips in the satellite link that an average reading would smooth over and hide. A reading of 65 means the worst Eb/No during that interval was 6.5 dB, while a 999 tells you the link dropped lock at least once in that window.