How are "Associations" used in the National Information Exchange Model (NIEM)?

The syntax for an instance of an association is simple. Take, for example, the marriage of Adam and Barbara Smith:

<MarriageAssociation>
   <SpouseRef s:ref="A"/>
   <SpouseRef s:ref="B"/>
   <MarriageDate>1937-05-12</MarriageDate>
   <DivorceDate>1973-06-02</DivorceDate>
</MarriageAssociation>

Interpreting the above XML fragment is straightforward:

These objects are specified elsewhere in the same XML instance: Object A is specified as follows:

<Person s:id="A">
   <PersonName>
      <PersonFullName>Adam Smith</PersonFullName>
   </PersonName>
</Person>

Object B is specified as follows:

<Person s:id="B">
   <PersonName>
      <PersonFullName>Barbara Smith</PersonFullName>
   </PersonName>
</Person>

Other elements in the association specify more information about the association:

<MarriageDate>1937-05-12</MarriageDate>
<DivorceDate>1973-06-02</DivorceDate>

The marriage date and divorce date are specific to the relationship between the two spouses, and so is a natural fit for an element of the association.