Hi Alan,
Taken straight from a website:
Copy backup. A copy backup copies all selected files but does not mark each file as having been backed up (in other words, the archive attribute is not cleared). Copying is useful if you want to back up files between normal and incremental backups because copying does not affect these other backup operations.
So it is basically a full backup anyway...the files that aren't backed up are the files that aren't selected. Otherwise anything in your selection list will continue to be backed up.
Laters!