The Additional Details page is provided for organizations that want to make add an additional page available to a project. For example, some Organizations use an 'Executive Summary' page. This page is not pre-configured with any fields. You associate the 'Additional Details' page with a project by way of the project category. When you configure a project category you can select the Additional Details section to include, as well as give it a different name.
You add fields to the 'Additional Details' page on Admin/Setup/All Entities/Projects/Additional Details. See Designing the Details Panel for information about how to configure the fields.