title: FCM Send message to multiple devices - which way to use?
body: <p>There are several ways to send FCM messages and I don't understand which one to use even if I read the documents over and over.</p>
  
  <p>My use case:
  I have a user that is logged into the app on her iPhone, iPad and Android tablet. I want to make sure the push notifications are sent to all of her devices.</p>
  
  <p>There are three ways to do this (I'm using the admin to send the push notifications):</p>
  
  <ul>
  <li><p><strong>Topic messaging (messaging().sendToTopic):</strong> My understanding this is for sending the same notification to larger groups. For example a topic could be all users that want weather notifications for New York. Not for my use case.</p></li>
  <li><p><strong>Send Multicast (messaging().sendMulticast([array of tokens]):</strong> My understanding this will send the same message to all device tokens in the array (up to 100). This seems perfect for my use case, right?</p></li>
  <li><p><strong>Device group messaging (messaging().sendToDeviceGroup):</strong> I don't really understand this one. Why use this instead of multicast or topics?</p></li>
  </ul>
  
  <p>For my use case it seems the best way is to each time a FCM token is updated I add this token to the user in the database. I then send the notification using the admin.messaging().sendMulticast([array of tokens]). Is this what you would recommend? Why or why not?</p>