The definition proposed by Grosz, Joshi, and Weinstein allows inferable entities, that is, entities that are not expressed at the surface level of the utterance or immediately recoverable from the subcategorization properties of the verb (as, for example, zero pronouns are) to constitute centers of an utterance.
the theory does not make explicit the parameters within which to characterize the class of permissible inferable elements or the constraints on doing so.