This is usually used with the fill to backstitch command in order to fill a pre-defined area of backstitch with a blackwork fill pattern. The Plus and Publisher editions come with over 100 blackwork fill patterns and more can be added by the user.
The motif browser will display the blackwork fills by repeating the basic pattern to fill the selected preview size. this allows the effect of filling a large area to be easily seen. The basic Professional edition does not have this extension to the browser.