Just fill in the name of a Vertex group in the Shape Panel, and this
Shape will then become blended with the reference Shape.
It is useful for example for a symmetrical modeled head, make a
copy of that Shape, and use two Vertex Groups to make it asymetric.
Of course the Shapes update nicely while Weight Painting.
Also new; since the Vertex group names reside on Object level, you might
want to copy these names to the other Objects that have the same Mesh.
That's a new button "Copy to Linked" in the first Edit Panel.
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