Infinite
By default, the influence area is set to Infinite. The entire art which is part of the Skin Group and which is not contained inside the Elliptical influence area of another bone, will move with the selected bone. If two bones have an Infinite influence type, then they influence the artwork up to where they meet, in other words, up to the joint.
Elliptical
You can set the influence area type for a selected bone to Elliptical using the Properties view. When set to Elliptical, an ellipse appears in the Camera view around the selected bone. An Elliptical influence type means that an ellipse shape determines the boundaries inside which art will be influenced by the selected bone animation. This area can be redefined using the Bone Influence tool or Properties view.