UID (Unique Identifier)

A globally-unique dotted-number string identifying studies, series, instances, and classes.

A UID is a globally-unique dotted-decimal string (e.g. 1.2.840.10008.…) that names a specific object. The hierarchy is Study → Series → Instance, each with its own UID: Study Instance UID, Series Instance UID, and SOP Instance UID. Restructuring or copying datasets means minting fresh UIDs so two objects never share an identity.