A signature in PDFClerk Pro terms refers to the number of sheets to be folded together to form a bindable section of a book. Normally one would not a apply a signature of more than 1 sheet for impositions other than 2-up, since the signatures for those larger layouts are usually the result of folding single sheets the required number of times and trimming them. But it is possible to specify multi-sheet signatures for such impositions if needs be; PDFClerk Pro will do the right thing when distributing the pages among the sheets. For 1-up layouts signature has no effect and is therefore not available. PDFClerk Pro will automatically calculate a magazine style global signature, encompassing all pages in the document, if this option is checked in the imposition template. The number of pages in a signature can be adjusted dynamically at any time through the sheets/signature input field in the toolbar.