Some posters above have missed the subtlety of the bit mask. In particular paradroid has missed that errorlevel 3 indicates a completely successful copy.
Note that bit 0x01 if set indicates that some files have been copied even if there were other failures. So any odd numbered errorlevels always indicate that at least some files have been copied.
Note also that bit 0x02 simply indicates that there are files at the destination that are not present at the source. This will happen if the /E switch is used and files have been deleted from the source since a previous copy was taken. It should not happen if the /MIR switch is used because that should delete files at the destination to mirror the source (but I haven't tested this).
So both errorlevel 1 AND 3 indicate successful copying of files with no errors.
Also errorlevels 0 AND 2 indicate that the destination is up to date and no files were copied.
For what its worth I came up with the following for my simple backup:
if errorlevel 16 echo Backup failed - see reason above & goto done
if errorlevel 8 echo All is not well - backup incomplete & goto done
if errorlevel 4 echo All is not well - some files were mismatched & goto done
if errorlevel 3 echo Backup completed successfully & goto done
if errorlevel 2 echo Backup already up to date - no files copied & goto done
if errorlevel 1 echo Backup completed successfully & goto done
if errorlevel 0 echo Backup already up to date - no files copied & goto done
I chose not to bother about the 'extra' files.
I have no idea what the 'mismatched' error is because it hasn't happened yet but I allowed for it just in case.