In XBOUND, a sheet corresponds to a sheet of paper – which consists of a front and (in some cases) a back page.
A sheet always has a front page.
A sheet is said to have a back page only if it is not blank.
A page always has at least one image.
A page may have two or more images associated with it in XBOUND (for example, a color image and a grayscale version).
See Adding a new page or Adding a new sheet.