This is a read-only value recorded in the call history table showing the coder rate that was negotiated for voice/fax compression on a now-completed call leg. An admin uses this historical record to determine, after a call has ended, which codec was actually used, useful for after-the-fact quality investigations. For example, an admin investigating a batch of complaints about a specific time window could review this field across several completed calls to check for a pattern of unexpectedly low-quality codec negotiation.