The Confidence value is a diagnostic that is normally reported as: 
-1     indicates that confidence not measured because it was not applicable
 0     indicates failure to verify checksum or ECC.  Some codes contain a checksum/ECC inherently which is always checked.   For some other codes, such as Code39, there is an option to use (or not use) a checksum.
100  indicates successful recognition
1-99 indicate a value used for diagnostic purposes

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