Three ways to split
Every page creates one PDF per page, which suits splitting a multi-sheet scan. By range creates one PDF for each range you write: 1-3, 4-6, 7- gives three files. Extract selected pages takes the pages you mark and combines them into one new PDF.
Writing ranges: separate them with commas, use a hyphen for a range, and end with a hyphen to run to the last page. Example: 1-3, 5, 8-.