It includes the cases in which the Cb of U i−1 might be "?," that is, that U i−1 might be the discourse-initial utterance or might simply have no Cb, while U i does have (an explicit) Cb.
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.