2.2.2.5 Background Page
A background page is a drawing page that can appear behind foreground pages and other background pages in a web drawing.
A background page can have a different drawing scale than a foreground page.
A background page in a web drawing is specified by the Page_Type element associated with the page.
If the Page_Type element associated with the page contains a Background attribute equal to one, it is a background page.
A background page in a web drawing has zero or one background pages as specified by the BackPage attribute of the Page_Type element associated with the page.