Demo URL : If your product (for example you are selling Joomla extension) has a page to show demo to customers, you can enter demo link into that field. When users browse for documents, they will see a demo button. When they click on that button, they will be redirected to demo page to understand all features of the product (document) before deciding to buy. If your document has no demo page, simply leave this field empty.
Detail URL : Normally, documents seller has a page to display detail information of a document. However, if you want to use a different page (not Documents Seller document detail page - for example, you want to write an article to describe more detail about your product), you can enter url of the page into that field. Then when users click on detail button to view detail information of the document, they will be redirected to that page instead of Documents Seller detail page.
Hope my explanation is clear to you
Regards,
Tuan