Extensions can be in independent *.xem files that are attached to models or can be embedded in model files. Independent extension files can be referenced by multiple models, and any changes made to such a file are shared by all models that attach it. Changes made to extensions embedded in a model file affect only that model.
You can attach an extension file (.xem) to your model at the creation of the model by clicking the Select Extensions button on the New Model dialog. You can subsequently attach an extension file to your model at any time from the List of Extensions.