On this tab, you can see a list of all documents of the currently edited document type. Using the Site drop-down list, you can choose if you want to display all documents of the type in the system or only on a particular site.
Detailed information about document types and transformations can be found in Developer's Guide -> Development -> Document types.
Page url: http://devnet.kentico.com/docs/5_5r2/contexthelp/index.html?doctype_documents.htm