In netCDF, you can create an attribute for the variable (and of the same type as the variable) called _FillValue that contains a value that you have used for missing data. Applications that read the data file can use this to know how to represent these values. Most netCDF readers disregard fill values and do not visualize them.
In addition to the fill value, selected variables have a dedicated value with a special interpretation. The fill value is not typically visualized, but the dedicated value is visualized.
The following variables have both the fill value and a dedicated value.
No detection, for example, there are no clouds to report when the sky is clear.
The device operates normally.
Reported in error situations instead of measurement data.
All other variables are assigned a numerical
value under normal operation and fill value
-999 in error situations.