If a change relies only on the control itself, like a change of a property (for example rename), the needed change handler has to be registered directly for this control type.
Changes that influence the aggregation of another control (for example move) have to be registered on the lowest upper-level control where the aggregation changes take place. This control is called the relevant container.
For example, for a GroupElement, the relevant container is the SmartForm where a move action across group boundaries is allowed. If the move action has to be restricted to a group, this group control is the relevant container.