ref:Chapter 

For a publication that uses chapters, this part should be used to capture this information.  Because chapters are not necessarily numbers, this is a string.

Element Information

Model

Source 

<element name="Chapter" type="string" substitutionGroup="link:part" id="ref_Chapter">
  <annotation>
    <documentation xml:lang="en">For a publication that uses chapters, this part should be used to capture this information. Because chapters are not necessarily numbers, this is a string.</documentation>
  </annotation>
</element>

Sample