Surface extrusion it's the equivalent of surgery Extracting the boss/base from solid modeling. Allows you to extrude a surface from a sketch in two directions.
However, a less known functionality of Surface Extrusion is the ability to create an extrusion on an existing surface from a wall.
You can extrude a planar face when selected with ALT or a 3D face.
For non-planar faces, remember the direction vector, which may be a line, edge axis or planar reference (as perpendicular).
You can close the end right away, remove the faces under the extrusion and merge the results. All options are clearly described in the operation properties.
