Maya uses the groupID node to represent sets.
Most sets in Maya are composed of a single node: the objectSet node. However, component sets are different: component sets are stored within the shape data instead of the object Set node. Each shape is connected to a groupId node that stores a unique identifier (ID) in the scene for the membership of the set within a shape's list of sets.
When deforming objects, you can use a componentTag to determine what subset to deform instead of the traditional way of deforming with groupId/groupParts/objectSet nodes. See Assign Component Tags to Geometry for more information about componentTags and membership.