The way the new_record.jsp page is set up, users must be very careful not to enter an ID number that is already in use, because each record in the database must have a unique ID number. This was explained in more detail under the note "Prevent duplicate keys" in Lesson 1.5: Program the Submit button. In this lesson, you will check to see if the ID entered is unique, and if not, send the user to an error page that describes the problem and tells the user how to fix it.
Navigation rules allow you to direct users to the error page or the all_records.jsp page after checking to see if users have entered a duplicate ID number or not. You will assign aliases to these two possible outcomes and then link those two aliases to the correct target pages. In this example, an error on the create_record.jsp page will signal the ERROR_CREATE alias, which will send users to the error page. If users fill out the create_record.jsp page correctly, it will signal the MAIN alias, which will be linked to the all_records.jsp page as usual.