The error function in Informatica can be very useful when you want to know why you have failed rows without failing the whole process. The use of the function is ERROR(string). Calling this function will make the error appear in the session log and the row will be ignored. For example: IIF(my_date_field > sysdate, ERROR('Futuristic Date'), my_date_field). This will ignore futuristic dates and show them only in the session log. By the way, you can declare default value for the port and the process will replace the ERROR with the default output.