Six types of special values may be displayed. 
...and 0 (unknown)  Data not available. 
<0.5 and 0 0 (negligible)  More than zero but less than half of the unit used. 
0 and 0 (zero)  An actual null value. 
F (FAO estimate)  Data estimated from available source of information or calculation based on specific assumptions. 
R (repetition)  Repetition of data previously reported. 
W (wrong unit)  Data for the record is in a unit that does not match the standard dataset unit. 
Other explanations 
no (number)  Indicates that data unit is number. 
t (tonnes =1000 kg)  Indicates that data unit is tonne. 
Tip:
when calculating, the program will preserve special values to the fullest extent possible, e.g. the sum of two unknown values is an unknown value, the sum of two negligible values a negligible value. In complex cases, all flags are removed and the result of the calculation is considered a "normal" number.
