The XSD schema used to denote suggested units as 'pages, minutes, etc.'  The outline of elements and attributes still does. Chris Putnam's bibutils had to address this almost a decade ago. Postel would tell you to import both and to export only 'pages'.

Also note that we've also talked about how to represent a reference that is on a single page.  Don't know where that is now, but I remember Chris had changed bibutils to emit:
  <detail type="page"><number>3</number></detail>

rather than your example:
  <extent unit="pages">