Please explain the CDA entryRelationship element

I have access to the HL7 Clinical Document Architecture, Release 2.0, which says that it is used essentially to link records to each other in a CDA document. In particular, he links between what is called the "source" and the "target" elements. I also read about various types of relationships (CAUS, COMP, GEVL, MFST, REFR, RSON, SAS, SPRT, SUBJ, XCRPT) and I understand them a little.

My main question: what are the "source" and "target" elements? Are they an element containing entryRelationshipand an element contained in entryRelationship?

For instance:

<entry typeCode="DRIV">
    <act classCode="ACT" moodCode="EVN">
        ...
        <entryRelationship typeCode="SUBJ">
            <observation classCode="OBS" moodCode="EVN">
                ...
                <entryRelationship typeCode="REFR">
                    <observation classCode="OBS" moodCode="EVN">
                    ...
                    </observation>
                </entryRelationship>
            </observation>
        </entryRelationship>
    </act>  
</entry>

, , SUBJ act observation, observation s REFR. ?