Use the Relationship Editor to edit relationships in Maya, where a relationship is a collection or grouping of objects or components. These relationships include:
When you are editing a container-like relationship, such as sets, partitions, or characters, the plus and minus icons add or remove the current selection from the highlighted container in the editor.
For example, if you are editing sets, you can highlight a set on the left side of the editor and click the Plus icon to add the currently selected objects to the set.