title: What is the purpose of `returningItems` in `NSExtensionContext.completeRequest`?
body: <p>I'm working on a standard Action Extension in my iOS app and the Xcode template contains the lines:</p>
  
  <pre><code>@IBAction func done() {
      // Return any edited content to the host app.
      // This template doesn't do anything, so we just echo the passed in items.
      self.extensionContext!.completeRequest(returningItems: self.extensionContext!.inputItems, completionHandler: nil)
  }
  </code></pre>
  
  <p>Here is the <strong><a href="https://developer.apple.com/documentation/foundation/nsextensioncontext/1411301-completerequest" rel="noreferrer">documentation for <code>completeRequest</code></a></strong>.</p>
  
  <h3>Questions</h3>
  
  <ul>
  <li>What is the purpose of passing <code>returningItems</code> to <code>completeRequest</code>?</li>
  <li>Do apps actually receive edited content from an action extension? </li>
  <li>If yes, where can I find the API on the receiving end?</li>
  <li>What are the consequences for me to pass an empty array?</li>
  </ul>