5.3.4 XML ID
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### 5.3.4 XML ID

The XML Specification allows a markup tag to have an attribute of type ID, whose value is unique within the document, that allows reference to that markup. The CDA schema defines such attributes with attribute name ID.

### Note

1. Thus the attribute named ID is of XML attribute type ID. This must further be distinguished from the element named id of HL7v3 Data Type UID that is part of most RIM classes. The attribute name is always upper case, the element name is always lower case.

2. The actual CDA schema specification uses the XML Schema Datatypes definition of XML ID (xs:ID). Readers may also be familiar with the xml:id specification, which is not formally used by CDA as it was published after the CDA specification.

In the CDA R2 Specification, the XML ID attribute capability is applied to the Section and observationMedia elements, and to various types of narrative block markup, and is used to provide linkage between structured entries and the corresponding narrative text (see Section 9.1.1 Section Text).

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