The is operator indicates whether the first of two operands is in the category of the second. The not operator indicates the opposite, that is, whether the first operand is not in the category of the second. The meaning of the operator depends on the left side of the comparison.
Use the is or not operator to check for I/O errors. If you are in V6 exception mode (see v60ExceptionCompatibility), you can compare a record (or record array) to both hard and soft errors. In normal exception mode, you can compare only to soft I/O errors; hard I/O errors will throw either a FileIOException (file I/O) or an SQLException (SQL I/O). For a complete list of I/O errors, see I/O error values.