Volumes are useful when you need to start numbering chapters anew in different sections of your book. Each Volume comes with its own Table of Contents and a Title Page which can be customized with title, subtitle, and author information.
Parts, on the other hand, allow you to create distinct segments within chapters while maintaining consistent chapter numbering. Parts will include a title page, which can be customized with the title and subtitle, but do not include a separate Table of Contents.