Walls are always perpendicular to the floor, so that the further information required are:
A plane is a flat surface which can be in a position with respect to either x - axis, y - axis or z - axis.
Generally in buildings, floors are usually along the horizontal plane that is parallel to the surface of the ground. And walls are along the vertical plane perpendicular to floors .
So that in the given question, the floor is on the horizontal plane, and the walls would be vertical to the floor at the marked points. Further information to specify the plane that will represent the walls are:
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