A rectangle, expressed in points with the origin at the bottom left corner, defining the extent of the page meaningful content (including potential white space) as intended by the page's creator.
A rectangle, expressed in points with the origin at the bottom left corner, defining the region to which the contents of the page should be clipped when output in a production environment.
A rectangle, expressed in points with the origin at the bottom left corner, defining the visible region of default user space. When the page is displayed or printed, its contents are to be clipped (cropped) to this rectangle and then imposed on the output medium in some implementation-defined manner.
A rectangle, expressed in points with the origin at the bottom left corner, defining the boundaries of the physical medium on which the page is intented to be displayed or printed.