This read-write enumeration (enabled or disabled) turns Error Correcting Mode on or off for T.30 fax sessions relayed through T.38, a feature many modern fax machines use to improve image quality and compression via reliable retransmission. An admin sometimes disables it specifically when fax calls are failing on a link with high packet loss, since ECM's own retransmission behavior can interact poorly with an already-lossy path. For example, an admin seeing repeated fax failures on a congested WAN circuit disables t38T30ECM to test whether removing ECM's retransmission overhead improves fax completion rates.