Imposition refers to how pages are laid out on a sheet of paper. PDFClerk Pro either shows the original pages exactly as they are (no imposition), or it shows pages as they will be imposed onto sheets of paper, according to the chosen imposition and sheet size settings.
Imposed layouts superimpose pages onto the sheets in a matrix of rows and columns. Each entry in the matrix is called a slot in PDFClerk Pro. A two up layout has either two columns and one row, or one column and two rows. Four up layouts will have two rows and two columns. The number of slots (= number of pages) on a sheet side is always a multiple of columns * rows, and the number of slots per sheet is always double that. So, a four up layout has 2 * 2 = 4 slots per sheet side, and eight slots per sheet.
Before explaining further, the following examples show how a factory provided imposition template affects the layout of the pages on onto sheets: A master PDF containing four single A4 sized pages, imported into PDFClerk Pro with default settings. The pages are imported without any alterations:
After selecting the default 2-up imposition template, the pages are laid out in pairs onto landscape sized sheets, and scaled down to fit the sheet’s paper size. Note that the status bar shows the name of the currently selected imposition template when one is active. It also shows the total number of sheet sides in the imposition: