List of 3D objects (T3D instances), that can be treated like another, larger 3D object.
It inherits from T3D class, so this list is itself a 3D object, representing a sum of all it's children 3D objects. This allows you to group many 3D objects, and treat them as one T3D descendant.
Additional child inside the list, always processed before all children on the Items list. By default this method returns Nil, indicating no additional child exists. The presence of this child can be calculated in overriden method using any condition, which is sometimes more comfortable than adding item to Items.
This item cannot be removed by methods like T3DList.Remove or by setting RemoveMe in it's T3D.Update implementation. Presence of this item is completely determined by GetChild implementation.