COHEs additionally have a field called s.nack_st, which indicates whether or not a NACK is an acceptable response to the coherence action. Specifically, a NACK is not acceptable if the coherence message demands a data transfer, as this indicates that the directory currently considers the cache to be the exclusive owner of the line. Thus, a value of NACK_NOK is specified for COPYBACK and COPYBACK_INVL messages, while NACK_OK distinguishes INVL messages.