This section is somewhat of a mix between two separate sections, Overriding VariantSets and Variant auto-selections - Using VariantSets to modify other VariantSets.
The idea is this - We can add variant selections from stronger layers and we can even use variant sets to modify other variant sets.
Can a variant set from a weaker layer modify a stronger layer?
Yes, it can
simple.usda shows the composition arcs without extra layers.
over.usda shows a weaker layer's overs being used to modify Prims defined in a stronger layer.
state_switch.usda uses a weaker layer to force variants in a stronger layer.
complex.usda demonstrates how to use weak variant sets to modify composition arcs in stronger layers.
This topic is very broad and has many applications.
Recall that in Understanding VariantSets it was mentioned that Variant Sets wrap the whole Prim, not just its children. You can exploit this behavior to do things like add overs, composition arcs, and other details onto a Prim.
- It's not a good idea to define new variant sets within weaker layers.
Check out nested_variant_example. It kind of works but kind of doesn't. For example, if you switch to geo=selection_1, it shows the look variant. But if you switch to geo=selection_2 then it disappears. Likewise, if you're using variant selection fallbacks then switching to look=look_1 will cause geo=selection_2 to be selected. In other words, selecting a variant in the "look" variantSet effectively erases the "look" variantSet's existence.
- Session selections will take priority over any "auto-change" behavior.
state_switch.usda shows this limitation clearly. The point of state_switch.usda is you're supposed to use </_container> and "some_other_variant=force_selection_1_in_the_geo_variant_set" to force "geo=selection_1" on the same Prim. But if "geo" already has a selection like "geo=selection_2", then that selection takes priority.
This same problem applies to any Prim which inherits from </_container>, such as </item_2>. If you change the variant selections of </_container>, it should apply to all child Prims like </item_1> and </item_2>. But if any of the child Prims were given their own selections then the auto-switch behavior doesn't happen for that Prim.
Depending on your scenario, this might actually be something you want. But if you're trying to rely on that behavior, this is definitely a problem you'd run into.