A Document shows a Layer tree structure. More precisely, it shows the layers of page zero first and then the layers of the current page. Page zero is reserved as a background page and shows on all other pages. Only one page can be modified at a time.
The background page (page 0) contains the grid and background rectangle which are modifiable on the Graphic View inspector editor. In addition, if you select page 0 via the Status Bar then the graphics of page zero can be altered directly and added to. That way you can add any arbitrary graphic to the background of any other page. Typical uses may include header and footer labels.
Since the background page is directly modifiable then you can also change most of the background and grid parameters directly. A good use would be to change the fonts of the grid axes. Notice that the grid and background graphic are locked and can not be unlocked so that they may not be deleted or move. Use the Graphic View inspector editor to turn the grid (or background) off and to synchronize those graphic attributes with the document parameters. Typically, the grid and background graphic are not to be edited directly.
Notice that the Navigator only shows the current page layers even though the Graphic View shows both page 0 and current page layers. That is because the background page is not modifiable while another page is the current page.