title: Magento: ?___SID=U appearing in some urls
body: <p>Can someone please explain why ?___SID=U is appearing in some Magento URLs on my site and not others?</p>
  
  <p>I think it has something to do with sessions but I am not entirely clear.  Also, what makes it more confusing is the fact that it's only appearing in some URLs and not others.</p>
  
  <p>I don't need to know how to remove it as I am aware of the setting in the admin area.  I would really like an explanation of what it is, what its purpose is and why it would show on some pages and not others.  id rather understand what's going on fully than blindly follow some advice as to how to remove it.</p>