Style-based numbering
A numbered or bulleted list may be associated with a style. It does not have to be. The recommended way, and the most stable approach to numbering, is to define it using Format>Style. If you do, your numbering will be associated with a style.
When numbering is associated with a style, Word sets a pointer in the style definition that links the style with a List Template in the document. The style does not store the numbering List Template; the style merely indicates which List Template to use.
This exposes us to the danger of circular references. If, say, your Heading 1 style points to Outline List Template 4, the List Template can point to nine styles, one for each level of the list. In fact, one of the hard and fast rules of defining outline List Templates is that you must never associate the same style with more than one level. Unfortunately, Word does not prevent this.
In a corrupt document, you often find that Heading 1 points to one List Template, Heading 2 to another, and Heading 3 to yet another. But each of the List Templates have nine levels, so each List Template points back at nine styles (a circular reference).