Content Items are identified by their position in the Content Item tree. They have an implicit order as defined by the order of the Sequence Items. When a Content Item is the target of a by-reference relationship, its position is specified in the Referenced Content Item Identifier (0040,DB73) in a Content Sequence Item subsidiary to the source Content Item.
Note: Figure C.17.3-3 illustrates an SR content tree and identifiers associated with each Content Item:
Figure C.17.3-3 Use of Position as SR Content Item Identifier (Informative)