How Extrude Between (3D Analyst) works
Each multipatch produced by Extrude Between represents the bounding volume for a given area between two surfaces. The surfaces provide the vertical constraints, and the polygon provides the horizontal constraint. The surfaces, due to differences and data area between themselves and the polygon, may play a secondary role in defining horizontal constraint.
First, the intersection between the interpolation zones of the two surfaces is made. This defines the common data area, in projection, between them. Then each polygon is intersected with this area. This defines its 2D boundary. It may be comprised of multiple inner and outer rings. The boundary is then profiled on each surface to get top and bottom height information. Extrusion between these two follows. The result is one or more vertical walls representing the side(s) of the output volume. The portions of both surfaces within the extent of the intersected polygon are extracted as discrete geometry and close the volume on top and bottom. The result is written as a multipatch to the output feature class.