The fourth mode is used control wall insertion, which controls the ability to insert objects in walls, including doors windows etc. If this mode is not activated, you will find it impossible insert doors and windows into walls and when you render the walls the doors and windows will not render correctly.
There are also times we want to turn this off. When you're moving a door or window along the wall it's good idea to turn this mode off so you do not accidentally pull the door and window out of the wall.
You might also want to turn this mode off when you're placing stairs, toilets, wash basins and so on.
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