The program auto-generates a large number of different fills, the default is 100 but you can type another number into the box (up to 9999). The program will continue to generate new fills, until it either reaches the maximum, or you click the Stop button.
A key feature of the program is the way in which it scores and ranks the fills it creates. They are scored according to a weighted average of various measures:  overall density of white squares (letter count),  word count,  number of checked squares - those shared by both an across word and a down word;  a measure of the overall area enclosed by 'word islands'. These statistics are displayed in a table which can be seen on the screenshot. While the program is running and new entries are added, the current 'best' fill is displayed at the top of the table - you can click on this at any time to update the displayed.
After the generation of fills is complete you can access any of them by scrolling the list. You can also try the Improve button, which looks for improvements by making minor changes to the currently selected fill.