Android FCM what is notification message and data message?

To send notification messages, set the notification key with the necessary predefined set of key options for the user-visible part of the notification message. For example, here is a JSON-formatted notification message in an IM app. The user can expect to see a message with the title "Portugal vs. Denmark" and text "great match!" on the device:

{
    "to" : "bk3RNwTe3H0:CI2k_HHwgIpoDKCIZvvDMExUdFQ3P1...",
    "notification" : {
      "body" : "great match!",
      "title" : "Portugal vs. Denmark",
      "icon" : "myicon"
    }
  }

Notification messages are delivered to the notification tray when the app is in the background. For apps in the foreground, messages are handled by these callbacks:

didReceiveRemoteNotification: on iOS
onMessageReceived() on Android. The notification key in the data bundle contains the notification.
See notification payload support for the full list of predefined keys available for building notification messages.

Data messages

Set the data key with your custom key-value pairs to send a data payload to the client app. Data messages can have a 4KB maximum payload.

For example, here is a JSON-formatted message in the same IM app as above, where the information is encapsulated in the data key and the client app is expected to interpret the content:

{
   "to" : "bk3RNwTe3H0:CI2k_HHwgIpoDKCIZvvDMExUdFQ3P1...",
   "data" : {
     "Nick" : "Mario",
     "body" : "great match!",
     "Room" : "PortugalVSDenmark"
   },
 }

On iOS, FCM stores the message and delivers it only when the app is in the foreground and has established a FCM connection. On Android, a client app receives a data message in onMessageReceived() and can handle the key-value pairs accordingly.

Note these further platform-specific details:

On Android, the data payload can be retrieved in the Intent used to launch your activity.
On iOS, the data payload is found in didReceiveRemoteNotification:.