A List Style stores the information about how to number each level.Only the List Style knows that Heading 1 is followed by Heading 2 and that it is followed by Heading 3.
A List Style 'groups' those paragraph styles into some order.
Word comes with built-in paragraph styles named Heading 1, Heading 2, Heading 3. A List Style creates a set or group of styles.And, each level stores information about how to number text to which that linked paragraph style has been applied. Each level can be linked to a paragraph style. If you edited the name of the heading, you’ll see that change take place both on the Ribbon and in the Styles menu.A List Style has 9 levels. Once you’ve made your choices, select “OK” to apply the modifications. If you want to learn more about how styles work, check out our guide to mastering styles and themes. If you’re unsure about any of these options, we recommend leaving them at their default settings. Since the template your document is based on is likely Word’s main normal.dot template, choosing this option usually means changes will apply to all your documents. You can also choose whether the style changes you’re making should apply only to the current document or should apply to new documents based on this template.
The next bit in the Modify Style window lets you choose whether to add the style to the Styles gallery and whether to automatically update the style if you make manual adjustments to a paragraph in your document that uses that style (we don’t generally recommend using that option because it always leaves people wondering how their style got changed).