Firebase send notification to specific user

It’s my first time using FCM. I download a sample from firebase/quickstart-android and I install the FCM Quickstart.

According to documentations, they’re unique , but you can’t bind them to a specific device since they might change.

It doesn’t expire though. It renews itself if one of the following happens. According to https://firebase.google.com/docs/cloud-messaging/android/client: -The app deletes Instance ID.