Is there a difference between a substitute check and an imaged check?

An 'imaged check' = Somewhere in the clearing process the check was converted to an image & the image clears through Fed and to the paying bank.
This did occur some prior to Check 21, but the image was followed up with the actual check going through the clearing channels and to the paying bank.

Check 21 allowed the check to be truncated & kept at the point where it was converted to an image.
Just printing "a copy of the image" does not create the substitute check; it has to be in the required Check21 format with the legal disclosure that says it is the equiv. of the original check.

A 'substitute check' / is created when someone takes the image and re-converts it back to paper = Prints the IRD (Image Replacement Document), the new document created under Check 21 that is the legal equiv. of the original check.