These two points belong together. Here you specify that a new document must be created at regular intervals and after how many pages this must be done.
Example: Let us assume that you scan invoices most of which consist of one page. By activating 'Create new documents' and entering '1' after 'Number of pages between documents' you can build a scan definition that allows you to put a whole stack of paper into the scanner: after each page a new document is created automatically. The scanned 'stream' of pages is thus separated into individual documents without the help of barcode technology.
If you want to scan invoices that come as two pages, you can create a second scan definition (and enter '2' beside 'Number of pages between documents).