@NA returns the value NA to indicate that a value needed to complete a formula is not available.
@NA is useful when you are building a worksheet that will contain data that you have not yet determined. Use @NA to flag cells where you will enter the data; formulas that refer to those cells result in the value NA until you supply the correct data.
@NA is also useful to determine which formulas depend on a particular cell.
@IF(B14< 0;@NA;B14) = NA, if the value in cell B14 is less than 0.