Add Bottom Wall Modifiers

Enables you to change the bottom height of a wall by trimming or extending it to a single-line object. The object can be a line, arc, or polyline. It must be at least as long as the wall, and cannot extend below the bottom of the wall. In addition, it must lie in the same workplane as the wall.

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If you add another Bottom modifier, it will cancel the effect of the previous one.
You can use Top and Bottom modifiers on the same wall.