From now on, Word will automatically redefine the style you selected whenever you apply manual formatting to any paragraph with this style.
Notes
This option is off by default, and it is recommended for experienced style users only.
This option is only available for paragraph styles (paragraph style: A combination of character- and paragraph-formatting characteristics that are named and stored as a set. You can select a paragraph and use the style to apply all of the formatting characteristics to the paragraph at one time.), and it is not recommended for the Normal style (Normal style: The default paragraph style used in documents based on the Normal template (Normal.dot).).