title: When and why to use &?, =?, @? in AngularJS?
body: <p>I followed some tutorials to create angular directives. In the isolate scope, some tutorials define scope like this:</p>
  
  <pre><code>scope: {
    model: '=?',
    data:  '@?' 
  }
  </code></pre>
  
  <p>Meanwhile, some tutorials define scope without question mark like this:</p>
  
  <pre><code>scope: {
    model: '=',
    data:  '@'
  }
  </code></pre>
  
  <p>Can anybody explain me the difference or the purpose of these with examples? Thank you.</p>