This is what the help files say about the NA function, which what you posted
is an alternate for (see the Remarks section)...
Returns the error value #N/A. #N/A is the error value that means "no value
is available." Use NA to mark empty cells. By entering #N/A in cells where
you are missing information, you can avoid the problem of unintentionally
including empty cells in your calculations. (When a formula refers to a cell
containing #N/A, the formula returns the #N/A error value.)
Syntax: NA( )
Remarks
===========
- You must include the empty parentheses with the function name. Otherwise,
Microsoft Excel will not recognize it as a function.
- You can also type the value #N/A directly into a cell. The NA function is
provided for compatibility with other spreadsheet programs.