Rigid skin point sets
During rigid skinning, for each joint, Maya creates a set of rigid skin points. The set contains all the points (NURBS CVs, polygonal vertices, or lattice points) that can be influenced by a particular joint. By default, the sets are organized into a partition, which means the sets can have no members in common.
You can organize points into a partition of sets before you bind skin, arranging for some of the points to become rigid skin points while other points can be influenced by, for example, a cluster or lattice deformer parented to a nearby joint. Using cluster or lattice deformers with rigid skinning can be a good way to get smooth deformation effects around areas such as shoulders, and provide an alternative to using flexors (see Flexors). However, you must plan ahead, and carefully decide which points are going to be affected by what, or your character could suffer from double transformation effects. See Double transformation effects.
For more information on sets and partitions see Sets and partitions.