What is the "Heads up" function on /e/ (quick settings)?

What does the “Heads up” function on /e/ (quick settings)? It is a ring bell symbol. It can be on or off. No extended option is show when the icon is pressed long.

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“Display priority notifications in a small floating window”

That’s what the description say.
How I found it out?
You can long tap on most symbols in the quick settings. This brings you to the corresponding real settings. In this case the “Notifications”. Here I scroll down, extend the menu and found Heads-Up.

Dear inubu,

Thank you for the explanation.

As said earlier the long press on the quick icon does nothing at my device. I’ll report a bug.
– Sent from /e/ Mail.

I totally missed your last sentence. I am sorry.

It’s not a bug. Many tiles don’t offer any explanation unfortunately.

You know when you get a message and it pops onto the top of the screen for a moment? That’s the heads-up notification!

Thank you for the notice.

I had the option set to on for some time and I had not noticed any change in behavior. Maybe no app would use the function.

I have reported it. Even if the option do not have extended options or settings, there is a description of the function in the Settings > Notifications.