When the "heading" field on a page is changed, the Table of Contents is automatically updated, since it shows the heading of the pages you specified using the "Add Topic" button. You are free to change the contents of this field for any page, including the Contents (cont) page and the Index (cont) page. That is, you can erase or replace the heading from these pages without affecting their operation. (Don't delete the field itself, though.)
Fields may be added as desired. The space at the bottom of each page is used for several "pop-up" fields and buttons, so avoid using it. Instead, you may add pop-up fields partially covering the main text field if desired, or simply add new cards.
Almost all the existing fields are named and are referenced in the scripts, so they should not be deleted. Be especially careful with the Contents* and Contents (cont) cards, which rely on the NUMBERING of the fields to work properly. You can confuse the scripts simply by deleting one field from these cards, or even by changing the field layering with HyperCard's "Send Farther" and "Bring Closer" options.