title: What's Page __EVENTARGUMENT?
body: <p>I saw code like this:</p>
  
  <pre><code>if (this.Request["__EVENTARGUMENT"] == "Flag")
     //DoSomthing...
  </code></pre>
  
  <p>What does <code>__EVENTARGUMENT</code> mean and are there some parameters like it to access?</p>