The numbers you select are the page number you wish to do a split on
As such selecting 1,3,7-8 would split a 12 page document into 6 separate PDFS with:
Document #1: Page 1
Document #2: Page 2 and 3
Document #3: Page 4, 5 and 6
Document #4: Page 7
Document #5: Page 8
Document #6: Page 9 and 10