When creating a key frame sequence (kfs2), you may want one rigid body to move as defined by another servo motor or key frame sequence (kfs1), but other rigid bodies to move as defined in kfs2.

For example, consider a window that rolls down as a car door opens. The door is referenced in kfs2, although the car is Ground. The window slides down relative to the door in kfs2. The car door body is Unspecified in kfs2. The car door is Required or Desired in kfs1 and closes relative to the car (Ground).