The ReferringURL property contains the URL that referred to the link which started the session.
ReferringURL contains the URL of the page that contained the link which started the session. This can be a start URL, or a URL which causes an implicit start. If started from a bookmark, manually, or from the StandAlone debug console, ReferringURL will be blank.
While ReferringURL may be better named ReferringStartURL, it was named before ReferringURLLast existed and has been left as is to preserve code compatibility.