I recently saw an excellent article by David Blatner describing the function of the slug area and how it is useful for attaching information to a PDF that sits outside the print area and crop marks. I thought he could also go on to explain that when exporting an InDesign file to PDF he could have mentioned that the pre-determined dimensions for the page size, bleed and slug areas are embedded into the PDF file and this allows anyone creating an imposition at the printer to place the pages to select the page area or page area and bleed area and to view then exclude the slug area from the imposition. This is important as it seems to be lost knowledge as most artwork studios draw crop marks in Illustrator!!!