title: What is the context parameter used for in Key value observing
body: <p>What's the use of context parameter in following method which is used  to register for key value notifications. The documentations just denotes it as arbitrary set of data. </p>
  
  <pre><code>addObserver:self forKeyPath:@"selectedIndex" options:NSKeyValueObservingOptionNew context:nil
  </code></pre>
  
  <p>Can somebody shed some light what's the purpose behind it ...</p>