UIDs are fundamental to DICOM and have many uses, including there being a unique one for each SOP_Class and for each Composite_Instance and each Normalised_Instance.

There are several rules for how a UID is constructed:

• It must be 1-64 characters
• Only 0-9 & “.” allowed
• No unnecessary leading zeros are permitted, so 1.2.826.0.1.012345.11.22.33 would be wrong
• No double full stops
• There must be a system to ensure global uniqueness
• UK companies have a root by rights, which is: 1.2.826.0.1.[English company number]

Medical Connections hands out free UID roots on request - see /FreeUID