CVN is the checksum verification number
It's used during emmissions checks or by dealers to determine if the contents of the ECU or engine computer are stock and un touched.
Each calibration of the ECU each firmware revision will have a different CVN
So the CVN must match the CVN for CALID calibration identifier issued by the manufacturer.
If the CVN is not correct for the corresponding CALID then the ECU contents have been changed or altered
Most scan tools can read the
CVN
CALID
VIN.
See here for further info
https://www.ft86club.com/forums/showthread.php?t=130030