Unique identifier of this instance of the clinical document.
Following attributes can be used to uniquely identify the instance:
@root: A unique identifier that guarantees the global uniqueness of the instance identifier.
The root alone may be the entire instance identifier. Required if @nullFlavor is not
present.
@extension: An optional character string as a unique identifier within the scope of the identifier
root.
The @root attribute must be a UID. The allowable formats and values and procedures
of this data type are strictly controlled by HL7. At this time, user-assigned identifiers
may be certain character representations of ISO Object Identifiers (OID) and DCE Universally
Unique Identifiers (UUID).
UUIDs SHALL be represented in the form XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX, where each
X is a character from the set [0-9a-zA-Z].
OIDs SHALL be represented in dotted decimal notation, where each decimal number is either
0, or starts with a nonzero digit. More formally, an OID SHALL be in the form [0-2](\.(0|[1-9][0-9]*))*.