The canvas is the area on which you draw diagrams. It usually reflects the size of a sheet of paper. It can be portrait or landscape and of any dimensions supported by your printer driver.
However, the canvas can also be extended to span multiple pages arranged in a poster grid. The poster grid is useful for creating very large diagrams that require more than one page. And it can also be used to divide a large diagram into multiple page sheets without having to divide the diagram between files.