In the case of positive acknowledgments (or negative acknowledgments to coherence messages sent with NACK_OK), the directory sees if this response enables it to send out a REPLY (if the access is not a cache-to-cache transfer and all coherence replies have now been collected). If so, the directory attempts to send out a reply, adding the reply to its OutboundReqs structure if no space is currently available in its port.
If the positive acknowledgment is an acknowledgment from a cache-to-cache transfer (whether it includes data or not), the buffer entry for the access is freed, as these accesses do not require any further messages to be sent.