If the document contains pages of various dimensions, 3 exclamation marks are placed in front and after the shown dimensions of new document. Whenever you see these exclamation marks, please be very careful to prevent unwanted imposition.
Get quick information about page dimensions by creating Report.
If USE CROP BOX options is checked, the Crop Box is taken, otherwise Trim Box is taken as page size.
Sometimes a combined PDF may contain pages from different page layout software (e.g. 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. You can set the sensitivity in PLDA Page/Settings dialog. The default sensitivity makes sure that A4 format both from Adobe InDesign and QuarkXPress is recognized as of the same size.