Beautify Cave Wall
Bottom: beautified.
Beautify Cave Wall is a command players can use to have citizens redesign the edges of cave walls that were previously dug out. When this command is selected, the player can make a selection by clicking and dragging around cave walls, but only cave walls.
When cave walls are dug out, the walls will have a rugged, rough look to them, but once they are beautified with the command, the walls appear much smoother and more aesthetically pleasing to the eyes of the player.
This is purely aesthetic though, as any race has preference for beautified walls and it doesn't increase happiness of your citizens.
In-game description
"Polishes cave walls and makes them more appealing to your subjects."