What is this icon with a square border containing lines that look like text?

When I use the volume keys on my Samsung Z Flip4 I get the volume bar for media/ringtone volume. When I tap on **...** I get the following advanced volume settings menu: What is the purpose of the small icon (marked red) on the top left? If I tap on it its gets strikethrough. Another tap reverses that, and so on. Seems like a toggle button for something but I cannot figure out its purpose.