The data in each IDC version, then, can be thought of as some set of versioned DICOM instances, series and studies. This set is defined in terms of the corresponding set of instance UUIDs, series UUIDs and study UUIDs. This means that if, e.g., some version of an instance having UUID UUIDx that was in IDC version Vm is changed, a new UUID, UUIDy, will be assigned to the new instance version. Subsequent IDC versions, Vm+1, Vm+2, ... will include that new instance version identified by UUIDy until that instance is again changed. Similarly if the composition of some series changes, either because an instance in the series is changed, or an instance is added or removed from that series, a new UUID is assigned to the new version of that series and identifies that version of the series in subsequent IDC versions. Similarly, a study is assigned a new UUID when its composition changes.