An animation set can be based on the current selection in the Scene View, or on the current selection set or current search set.
When you add an animation set based on a selection set, the contents of the animation set is automatically updated if the contents of the source selection set change.
When you add an animation set based on a search set, the contents of the animation set is updated each time the model changes to include everything in the search set.
If the model changes so that objects in a particular animation are missing, they are automatically removed from the animation set when the corresponding NWD or NWF file is resaved.
Finally, if the selection or search sets are deleted rather than being lost, the corresponding animation set becomes a static selection of objects based on what it last contained.