The scope of Firebase Instance Id and Token

Question

I've read a lot of articles about Firebase Instance Id and registration token here, here and here.

I have some confusions about their scope and relationship between Instance Id and Token. I would like to validate that my understandings are correct or wrong.

1. Instance Id is generated per App. Not per installed device. For example, if you have Mail app and Notes App, you will have two Instance Id on firebase.
2. Token is generated per installed devices. For example, if you have ten users installing your app, you will have ten registration tokens.
3. One Instance Id will have many registration tokens.
4. Normally, we only have to keep registration tokens (not Instance Id) on our App Server to send push notifications.

Answer 1

1. Partly correct. An InstanceID pertains to the app instance. In your example, if you have the Mail App and the Notes App, each has their own instance id, but the two are not the same, so they can't be grouped together.

   Think of the app instance as the app installation. If you have installed the app on the device, it has the app instance. If you uninstall it, it is also removed, if you re-install it, it's a new app instance.

2. Correct. But there's also a possibility to have multiple tokens for the same device, but this only pertains to having multiple senders.

3. Possible. See #2.

4. Yes. The registration token is the value needed as a target for push notifications.