Subpages are the smallest entity - they can contain one chart, e.g. contours superimposed on a background map of coast lines and grid lines.
A geographical projection or cartesian transformation is always attached to a subpage.
One or more subpages can be plotted on a page, provided that they fit within the limits assigned to that page.
Subpages can be of varying sizes. A subpage is defined by the position of its bottom left hand corner and its lengths in the horizontal (X) and vertical (Y) directions.
The position of the subpage is relative to the bottom left hand corner of the page.
Subpage define the projection.