Not A Number/Path/Refnum?

Returns TRUE if number/path/refnum is not a number (NaN), not a path, or not a refnum. Otherwise, this function returns FALSE. NaN can be the result of the square root of a negative number. The connector pane displays the default data types for this polymorphic function.

number/path/refnum can be a scalar number, path, or file refnum, or it can be a cluster or array of numbers, paths, or refnums.
NaN/Path/Refnum? is a Boolean value of the same data structure as number/path/refnum.