Volume sound widget

I use Ubuntu 16.04 LTS Unity DE. When pressing F1, F2, or F3 the sound volume control will be displayed on the top right screen showing the current volume status like below:

https://imgur.com/a/GyNQj

I am wondering how this is triggered or get displayed? The reason I ask is because I want to display such status in other environment like i3 wm, but I do not know how it works. Any doc or pointer to this?

Thanks

Hello,

I think its just a unity specific widget to shortcut the system volume of your system. I watched a bunch of instructional videos and i believe that was mentioned.