A Composite Instance is the official DICOM term for a persistent storable object. Whilst most composite instances are Images (with pixel data), DICOM has plenty of other Composite Instance types, such as:

• Structured Reports
• Key Image Notes
• Presentation States
• Radiotherapy Objects

Every object in DICOM, whether a permanent Composite Instance or merely a transient Normalised Object as used in Printing has a unique UID called its Instance UID. This is different from the SOP class UID which is shared with all other objects of the same type.