Extrusion is typically the first step to generate a 3D building from a 2D footprint. This operation increases the dimension, i.e. a two-dimensional building footprint can be extruded to a three-dimensional mass model, see extrude operation in CGA reference.
Example
Lot --> Lot. A 2D building footprint in its initial scope (yellow). The scaled x- and z-axes of the scope are illustrated in red and blue respectively. The y-dimension of the scope is zero. |  |
Lot --> extrude(4) The 2D building footprint is extruded to a 3D mass model. The extrusion direction is orthogonal to the shape (along the normal of the footprint polygon). The scope has changed. In this example the x- and z-dimensions are the same but the y-dimension of the scope has changed to the extrusion height. If a footprint lies on a hill, it might be desired to extrude along a world coordinate axis rather than the polygon normal. This and other extrusion variants are explained in the extrude operation. |  |