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The easiest way to do this is to use Styles.
You can give the Part X a different heading style to the numbered chapters.
For example, here I have created Part 1 using the “Chapter” style then the following chapters use “Chapter Numbered”. This is how the Navigator looks with these styles:
The numbered chapters will follow on sequentially every time you use that style.
You can create your own custom style by clicking on the icon
You can also format the pages differently. So, for example if you want a blank page with no headers or page numbers, just the “Part X” text, you can use Master Pages to do this.
To create a new master page layout, go to Document -->Page Layout --> Master Pages
Click “New Master Page”. You can base the new one on the standard if you want. Save it with a new name. Edit the page layout to the format you want.
Once you have created your new master page, go to “Assign Page Range” and select which master page relates to which page.
For more information on this, take a look at our Wiki articles on Styles and Master pages.