Data messages vs Notification messages, (collapsible and non-collapsible confusion)

Data messages:

Docs says:

Non-collapsible: Every message is important to the client app and needs to be delivered. Except for notification messages, all messages are non-collapsible by default.

that means Data messages are non-collapsible and are considered important. However, docs also say:

Data only messages are considered low priority by devices when your application is in the background or terminated, and will be ignored.

So, are they actually important or not and if they are, then how come they be ignored if the app is in the background, for instance.

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Notification messages:

Docs says:

Except for notification messages, all messages are non-collapsible by default.

I'm sending two notification messages through the Firebase Notifications composer and being collapsible, old message should collapse (or replaced by the latest one) but I can see both of the notifications being displayed on both Android and iOS.

So, how come Notification messages are considered "collapsible"?

from Data messages vs Notification messages, (collapsible and non-collapsible confusion)