You can represent an unstructured document in CDA as either C-CDA (Consolidated CDA) or HITSP C62. C62 is much more commonly supported today; a quick GitHub search does not show any unstructured C-CDA implementations.
Note:the references and examples below are from non-normative specifications. You will probably need an HL7 membership to view the normative standards.
C-CDA:
From MDHT Models documentation (account required):
SHALL contain exactly one [1..1] templateId ( CONF:7710, CONF:10054 ) such that it
a. SHALL contain exactly one [1..1] @root="2.16.840.1.113883.10.20.21.1.10"
Example
<?xml version="1.0" encoding="UTF-8"?>
<ClinicalDocument xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="urn:hl7-org:v3" xsi:schemaLocation="urn:hl7-org:v3 CDA.xsd">
<realmCode code="US"/>
<typeId root="2.16.840.1.113883.1.3"/>
<templateId root="2.16.840.1.113883.10.20.21.1.10"/>
<templateId root="2.16.840.1.113883.10.20.22.1.1"/>
<code code="18842-5" codeSystem="2.16.840.1.113883.6.1" displayName="Discharge summarization note"/>
<confidentialityCode codeSystem="2.16.840.1.113883.5.25" codeSystemName="ConfidentialityCode"/>
<custodian>
<assignedCustodian>
<representedCustodianOrganization/>
</assignedCustodian>
</custodian>
</ClinicalDocument>
HITSP C62:
From MDHT Models documentation (account required):
SHALL contain exactly one [1..1] templateId ( ) such that it
a. SHALL contain exactly one [1..1] @root="2.16.840.1.113883.3.88.11.62.1"
Example
<?xml version="1.0" encoding="UTF-8"?>
<ClinicalDocument xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="urn:hl7-org:v3" xsi:schemaLocation="urn:hl7-org:v3 CDA.xsd">
<realmCode code="US"/>
<typeId root="2.16.840.1.113883.1.3"/>
<!-- HITSP C62 template -->
<templateId root="2.16.840.1.113883.3.88.11.62.1"/>
<!-- HL7 General Header Constraints-->
<templateId root="2.16.840.1.113883.10.20.3"/>
<!-- IHE Medical Documents -->
<templateId root="1.3.6.1.4.1.19376.1.5.3.1.1.1"/>
<!-- IHE Scanned Documents (XDS-SD) -->
<templateId root="1.3.6.1.4.1.19376.1.2.20"/>
<code code="18842-5" codeSystem="2.16.840.1.113883.6.1" displayName="Discharge summarization note"/>
<recordTarget>
<patientRole>
<patient/>
</patientRole>
</recordTarget>
</ClinicalDocument>
You can view some additional XML examples in the MDHT automated test results.
For a receiving organization to differentiate the content of an unstructured document, you should store the content type in the <code> element as shown in the examples. The content type can also be stored in the <classCode> or <typeCode> elements in an XDS submission set.