<surface> element represents a page, or any other inscribed surface.
An encoder can choose to encode a double-page spread as either a single
<surface> or as two distinct
<surface> elements each representing a single page.
<surface> element contains information about any rectangular regions of interest on that surface, and any graphics which depict it. A
<surface> may bear attributes, which align it with particular areas of interest on that surface.
(Wittern et al. 2009, 290)