What does "stacked panes" mean?

[Bud]

Your ArcGIS Pro Update (November 2024)

Stacked Panes with Icons
Stacked panes now feature icons, making it easier to recognize which pane you need without having to hover over or click each tab. 

What does "stacked panes" mean? When would panes become stacked?

[Robert_LeClair]

So stacked panes are seen, typically, on the right side of the application where the Catalog Pane.  As you open more panes, they're stacked on top of each other.  Icons were added to the names of the tabs to make it easier to find/identify.  The drop down arrow, in the image right side, is a menu open tabs that you can click and navigate to.

[Bud]

Are the panes always stacked horizontally, never vertically?