Document Product Type is used for Documents that are also available for sale.
You might wish to display brief peices of the Document then offer it for sale.
This Product Type is also handy for downloadable Documents or Documents available either on CD or by download.
The Document Product Type could be used in the Document Sidebox or the Categories Sidebox depending on how you configure its master categories id.
This product has also been added as a linked product to the Document Category. This will allow it to show in both the Category and Document Sidebox. While not marked specifically for the master product type id related to the Product Types, it now is in a Product Type set for Document General so it will show in both boxes.