The Client Composition Extension is a Java class that allows operation-specific code to run in the context of the PE Client Composer. The Client Composition Extension is specified by the parameter e3.clientCompositionExtension. The parameter must be of type local, and its value should be a fully-qualified Java class name. The class must implement the interface com.arbortext.e3.ClientCompositionExtension.

The PE Client Composer will instantiate an object of this class once, the first time it is referenced. After the initial use, the Client Composer will save the object in a map and reuse the same object for subsequent publishing operations that specify the same Composition Extension name. A Composition Extension must be serially reusable. It does not need to be thread safe, however, as Arbortext Editor can not execute simultaneous publishing operations.