A Composite IOD is an Information Object Definition which represents parts of several entities in the DICOM Model of the Real-World. (See PS 3.3.) Such an IOD includes Attributes which are not inherent in the Real-World Object that the IOD represents but rather are inherent in related Real-World Objects.
These related Real-World Objects provide a complete context for the exchanged information. When an instance of a Composite IOD is communicated, this entire context is exchanged between Application Entities. Relationships between Composite IOD Instances shall be conveyed in this contextual information.
Notes: 1. Actual communication of IOD Instances is via SOP Instances.
2. Whenever Composite SOP Instances are in fact related, some of the contextual information is redundant (i.e. the same information about the same Real-World Objects is contained in multiple SOP Instances).
The Composite IODs are specified in PS 3.3.