title: Analytics.js: What does 'auto' argument do on tracking object creation?
body: <p>I've just switched to the new Google Analytics <a href="https://support.google.com/analytics/answer/2790010?hl=en-GB" rel="noreferrer">Universal Analytics</a> platform, using the new javascript API -  <a href="https://developers.google.com/analytics/devguides/collection/analyticsjs/" rel="noreferrer">Analytics.js</a></p>
  
  <p>In the first example code snippet from <a href="https://developers.google.com/analytics/devguides/collection/analyticsjs/" rel="noreferrer">the docs</a> a tracking object is created, then a pageview of the current page is logged using that tracking object.</p>
  
  <pre><code>ga('create', 'UA-XXXX-Y', 'auto');
  ga('send', 'pageview');
  </code></pre>
  
  <p>Notice the <code>'auto'</code> argument - <strong>I can't work out what that's there for</strong>.</p>
  
  <p>I had a look at the <a href="https://developers.google.com/analytics/devguides/collection/analyticsjs/method-reference#create" rel="noreferrer">method reference</a> and it seems that the relevant parameter is optional and should contain configuration key/value pairs. Examples can be found in the <a href="https://developers.google.com/analytics/devguides/collection/analyticsjs/field-reference#create" rel="noreferrer">field reference</a>. If the parameter is optional it seems odd to pass <code>'auto'</code> here unless it does something of significance, so you have to assume it does, but I can't find any mention of this.</p>
  
  <p>Also, in the auto generated code in your account it does a similar thing but passes your domain</p>
  
  <pre><code>ga('create', 'UA-XXXX-Y', 'mydomain.com');
  </code></pre>
  
  <p>The fact your domain is specifically passed when they have it available again suggests it does something meaningful - otherwise they'd probably just use <code>'auto'</code> here too. But again, there's no mention of why this is done.</p>
  
  <p>Am I missing something really obvious?</p>