Here’s how to group your chapters together into a Part or Volume:
Hold down the Shift key on your keyboard.
Select the chapters you want to include in the grouping. Just click on each chapter in the left navigation pane while holding the Shift key.
After selecting the chapters, simply right-click and choose either 'Create a Part' or 'Create a Volume' from the options that pop up in the context menu.
That's it! Atticus will take care of the rest and create your desired Part or Volume with the selected chapters.
*Note: You cannot create a Part inside of another Part; however, you can create a Part inside a Volume.
You can learn more about working with the Parts and Volumes features in this tutorial: https://www.atticus.io/how-to-format-your-book-with-atticus/