Say you have a few objects in a container, of which some are also in families.
If a condition picks one object in a container, every other associated object in its container is also picked.
But that doesn't allow you to pick them as a family instance without creating extra events that could end up complicated depending upon your implementation.
This behavior has a couple expressions where you provide the family name, and it will return the uid(s) of the sibling(s) in that family that are also in the container.
There is a capx included that shows a basic example of its use.