Drag & drop the object whose motion you want to transfer (the source object) into this box.
Into this box Drag & drop the object that should receive the motion (the target object). Only the highest object in the hierarchy must be dragged into this box — its child objects will be included in the transfer automatically.
This option, which is enabled by default, makes the Retarget Expression compare the hierarchical structure the of reference objects (source, target and neutral) including their children.
Objects that are in the same position in the hierarchies — such as the direct child of the top-most object in the hierarchy — will be assigned to one another and compared. This is assuming that the source, target and neutral reference objects all have the same hierarchical structure. For example, the hip bone must be in the same position in the hierarchy for all three reference objects.
If the option is disabled, the Retarget Expression will orient itself according to the names used in the hierarchy. This is useful when there are differences in the hierarchy between the source object and the target object. One example of this would be when transferring the animation of a character that has a tail to a character that doesn‘t have a tail. Because a tail does not exist in the character that is receiving the transferred motion, the tail motion data will be ignored. For this type of transfer to work, it is essential that you use a naming convention consistently for the characters involved in the transfer. In time, every animator develops his or her own routine for naming conventions and can make good use of these in order to marry bone hierarchies, simply by using the naming convention!