The Change_description can be used to define / highlight the actual content of a Change. Therefore the Change_description can reference a set of Changed_elements in order to highlight that these elements have been affected by the change (in a paper based process this would have been done with "red circles"). However, deleted elements can not be highlighted with this mechanism.
Since a physical change (the Change_description) can result in multiple changed parts (e.g. Modules) with different meta data, a Change_description can have a number of "related_changes". For example if a segment length is changed, all Modules that have wires through this segment will be changed. So the initial Change_description is the same for all Modules, but the meta data of the Change might be different for each Module (e.g. change index, approver, ...)