Brother MFC-640CW - Manual (Page 45)

INTRODUCTION 1 - 20
ECM (Error Correction Mode)
The Error Correction Mode (ECM) is a way for the machine to check
a fax transmission while it is in progress. If the machine detects
errors during fax transmission, it resends the pages of the fax that
had an error. ECM transmissions are only possible between fax
machines that both have this feature. If they do, you may send and
receive fax messages that are continuously being checked by the
machine.Your machine must have enough memory for this feature to
work.