Plane_Align
From ZDoom Wiki
181:Plane_Align(floor, ceiling)
floor: "1" slopes the floor on front side of the line; "2" slopes the floor on the back side. ceiling: Same as for floor, but for the ceiling plane, respectively. Note: A value of 0 means no sloping is applied.
Aligns the plane of the floor and/or ceiling depending on the values floor and ceiling. For a more extensive look, check out slopes.
An example WAD can be found at http://zdoom.org/files/examples/slopes2.zip

