Linear denotes whether the section is Primary or Auxiliary. Usually the cover page has linear set to false and all the other sections have it set to true. It's an optional property that readers may also ignore.
primary or auxiliary is useful for Reading Systems which opt to present auxiliary content differently than primary content. For example, a Reading System might opt to render auxiliary content in a popup window apart from the main window which presents the primary content. (For an example of the types of content that may be considered auxiliary, refer to the example below and the subsequent discussion.)