A set is a collection of objects or components. Any item you can select can be in a set. The set exists as a separate node representing the collection. Unlike groups, sets do not alter the hierarchy of the scene - they're simply an arbitrary collection. They are always saved at the scene level, and cannot be part of an object, group, or hierarchy.
You can create two different types of custom sets: sets and quick select sets. Both of them can contain selected objects, components, or groups, but a quick select set can't be added to a partition. As their name implies, quick select sets are most useful for making it convenient to select any collection of objects or components. For example, you can select many vertices on a polygon object and put them in a set so that you can select and model with them easily.
In some instances, Maya creates sets for you as you work with objects. For example, when you add a cluster deformer to some CVs of a NURBS surface, Maya makes a set for the CVs. You can edit the set to control the effect of the deformation. Maya also creates sets that represent shading groups and layers, and points controlled by deformers, flexors, and skin.
Sets are useful for the following:
You can control the membership of sets easily using either the Outliner or the Relationship Editor, as described in Create and edit sets.
A partition is a collection of related sets. Partitions prevent the sets in them from having any overlapping members. Maya uses partitions to keep sets separate where overlapping members could cause problems.
Maya creates partitions to keep character sets, shading groups, skin point sets, and exclusive deformers from having overlapping members.
You can create your own partitions when you want to create sets that have no overlap.
For example, suppose you’re animating a cartoon character’s nose as he smiles and laughs. You added a cluster to several CVs for adjusting the nose as he smiles and another cluster to different CVs for adjusting the nose as he laughs.
Creating the two clusters creates a set for each group of CVs. Occasionally you want to move CVs from one set to the other. When you move the CVs from one set to the other set, they remain in the first set. You might not want the CVs in the first set because they add undesirable deformations as you transform the cluster.
To avoid this problem, you can create a partition and put both sets in it. The partition prevents one set from having members of another set. When you move the CVs from the first set to the second set, they’re automatically removed from the first set.