Example: Extending a subject scheme upwards

The following subject scheme map creates a "Software" category that includes operating systems as well as applications. The subject scheme map that defines the operation system subjects is pulled in by reference, while the application subjects are defined directly in the subject scheme map below.

<subjectScheme>
  <schemeref href="baseOS.ditamap"/>
  <subjectdef keys="sw" navtitle="Software">
    <subjectdef keyref="os"/>
    <subjectdef keys="app" navtitle="Applications">
      <subjectdef keys="apacheserv" navtitle="Apache Web Server"/>
      <subjectdef keys="mysql" navtitle="MySQL Database"/>
    </subjectdef>
  </subjectdef>
</subjectScheme>

If the subject scheme that is defined in baseOS.ditamap binds the "os" subject to the @platform attribute, the app subjects that are defined in the extension subject scheme do not become part of that enumeration, since they are not part of the "os" subject

To enable the upward extension of an enumeration, information architects can define the controlled values in one subject scheme map and bind the controlled values to the attribute in another subject scheme map. This approach will let information architects bind an attribute to a different set of controlled values with less rework.

An adopter would use the extension subject scheme as the subject scheme that governs the controlled values. Any subject scheme maps that are referenced by the extension subject scheme are effectively part of the extension subject scheme.