A recent thread on the dita-users list discussed specialization as an extension of the DITA OT by restrictive substitution. This is best done by creating a DITA-OT plugin.

Plugins are overrides to the OT code that add functionality. Plugins are used to add new specializations to the OT and to define new specialization targets for output processing of the specialized elements. If they are designed properly, plugins can be added to an upgraded version of the OT and still run properly.

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