No rules at all. Headings should get just a unique integer value and better if it is more than 10000. So if the flow is
h1, h2, h2, h3, h1, h2, h3, h4, h5, h6, h2, h3.... then the headings should now be 10001, 10002, 10003, 10004, 10005, 10006, and so on. The numeric value can be any but better if it is at least > 100 or 1000.