I am not totally sure I understand your question but I will take a stab at it.
The action page is arbitrary - it's simply the page that will process the form after the submit button (or other trigger) is pressed.
Many times, developers use the exact same page that the form is on for the processing using PHP code at the top that determines if the submit button has been pressed and then processes the information and then redirects to another page after the processing.
For the action page, it would be set to nothing if you just want to use the page your form is on. If you want to handle the processing on a totally separate page, then you need to specify that page in your action page dialog.