When you drop an object into a Character Extension, the Translation and Rotation values of the object are added as reference properties in the Character Extension properties. A Label property is also added for any object added to an Extension, which lets you create a second non-unique name for the object.
The Labels that you define for each object in an Extension have a similar function to the body part names in the character Mapping List - they both let you describe a structure. The difference is, you can change the Label names for objects that make up your Character Extension.
For example, on a character with Character Extensions for two wings, you might want to mirror poses from the right wing to the left. To do this, you can create a symmetrical set up where the two Character Extensions are each made up of three objects. All of the wing objects must have a unique name in the scene, but you can indicate which objects correspond wing-to-wing by giving each pair the same Label, such as WingA, WingB, and WingC. Using the same Label for two objects indicates that the objects are compatible, so you can retarget animation between them, and mirror poses on them.
When you are ready to plot your character animation, turn on the Plot character Extensions option in the second dialog box. Also, use the Plot Method property to specify that you want Character Extensions to be plotted when you are plotting to the skeleton, Control rig, neither, or both.
Plotting Character Extensions works around a limitation where the Mapping list only offers you five slots for prop objects. (Anything in the Mapping list gets plotted along with your character.) Because you can add any number of prop objects to a Character Extension, you can now plot animation for any number of objects associated with your character.