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"><start>3</start></extent>