Animation Utility Objects
The movement of an animated group of surfaces is defined using one of the three utility objects: nulls, rotation axes, or translation vectors.
Nulls, rotation axes and translation vectors possess representations to help you to visualize them and to position them in the 3D space.
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Note
The mode 
, enabled by default, allows objects such as surfaces to be transformed when their parents are transformed. Deselect this mode in order to reposition objects that are already part of a parent-child relationship. This will prevent objects such as surfaces associated with a part from being moved when you reposition the representation of the null, axis, or vector.