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