The numbers you select are the page number you wish to do a split on

As such selecting 1,3,7-9 would split a 10 page document into 6 separate PDFS with:

Document #1: Page 1

Document #2: Page 2 and 3

Document #3: Page 4, 5, 6, 7

Document #4: Page 8

Document #5: Page 9

Document #6: Page 10