Reapplying

Check Your Options

You can reapply a style to a selection of text by using any of the methods described on the Applying Styles page.
However if you have the following option selected (Tools > Options)(Edit tab, "Prompt to update style") then you re-select the same style then a message box will be displayed instead.

Prompt to update style - Specifies that Word will prompt you when you re-apply the same style to a selection of text.
Using the Styles & Formatting task pane
Add some text.
Click on task pane style
Click on task pane style again to see the prompt
Does the style need to have any manual formatting ???
When this is not ticked the style is just re-applied to the selected text.

Modify Style Dialog Box

This dialog box will be displayed when you reapply the same style to a selection of text.
This lets you change a style without using the Modify Style dialog box (??)"
This allows you to make the formatting changes (character & paragraph) by re-selecting the name of the style.

Update the style to reflect recent changes - This will change the style selected to match the current paragraph.
Reapply the formatting of the style to the selection -
?? - Never tick this option.

what os the difference between "Modify a style" and "re-define a style")

Important

The Modify Style dialog box will not be displayed when you reapply the built-in Normal style.
The Reapply the formatting option should never be used to modify any bulleys or numbring