Custom pages are used to override the page templates defined in DynamicData\PageTemplates for specific tables.  E.g. to use custom pages for the Products table, you would create a DynamicData\CustomPages\Products folder, and create aspx pages in there.  One way to get started is to copy the pages from DynamicData\PageTemplates into DynamicData\CustomPages\Products, and then modify them to fit your needs.