Choose the language and units that suit your needs.
Units may be millimeters or inches.
Sometimes a combined PDF may contain pages from different page layout software (i.e. Adobe InDesign and QuarkXPress) and as you can see the page sizes even though they were set the same in the programs may differ (older versions of QuarkXPress are not precise). Nevertheless, these differences are not really important in most cases. Therefore there is an option to ignore such small differences. The default sensitivity makes sure that A4 format both from Adobe InDesign and QuarkXPress is recognized as of the same size.