In Physique, the envelope is the primary tool for controlling skin deformation. An envelope defines an area of influence about a single link in the hierarchy. If the envelope is deformable, mesh vertices within that envelope follow the movement of the Physique deformation spline. An envelope has a pair of inner and outer bounds; the envelope's influence is strongest at the inner bound, and falls off toward the outer bound. By default, each envelope has four cross sections. You can reshape the cross sections, or add new ones, to change the envelope's geometry.
In the Skin modifier, the envelope plays a similar role with respect to bones. Each bone has its own envelope with two or more cross-sections that allow you to shape the envelope to fit the surrounding mesh.
Typically, the envelopes of adjacent links or bones overlap each other. Vertices that fall in the overlap area are weighted to produce smooth blending at joint intersections.