What changes are tracked?

Changing any property on the tracked object either if you're setting its default value (for example null or an instance of a reference or value type).

Adding, removing or updating an item in a collection property on some tracked object.

As above case, with sets, an intersection or exception will also be tracked as adding or removing items to the collection.

Any of above cases on any level of associated objects with the one that was turned into a change-trackable object.