Check 21 refers to the Check Clearing for the 21st Century Act. Check 21 is a federal law that allows banks to handle more checks electronically. It allows banks to
truncate a check by removing the original from processing.
exchange images of a check instead of the original paper check.
create a special paper copy, called a substitute check, of an original check.
A substitute check is
a legal document, with all the rights and obligations of the original check.
a reproduction that includes all information and endorsements from both the front and back of the original check.
used as proof of payment.
More information on this topic can be found at Check Clearing for the 21st Century - FDIC.