Recommendation Details: The CF _FillValue attribute is used to indicate missing or invalid data for a variable.  Also, the value of the CF _FillValue attribute should match the actual fill value used for the variable in the file.

The value of the CF _FillValue attribute should be a mathematically valid number that lies outside the valid range for a variable.  Please note that NaN (Not-a-Number) is neither a number nor is it mathematically valid, and, thus, should not be used as the fill value (see Recommendation 3.7 of ESDS-RFC-036).

If possible, using zero as the fill value should be avoided, because zero looks too much like a physically realistic value, and this can be confusing to the end users.

There should only be one fill value per variable.  We recommend using a quality flag variable along with the CF flag_values and flag_meanings attributes to explain the various reasons for using the fill value, instead of using several special values in the variable.