EAN128IsFNC1OmmitedForFixedLength is true by default.
EAN128IsFNC1OmmitedForFixedLength omits FNC1 for fixed length fields (AIs). FNC1 can not be removed from other fields as reader wouldn't know where one field ends and the next one starts.
Barcode.dll has a build-in dictionary of fixed length fields (AIs).
e.g. (00), (01), (3100-3690) and others.
You can mark specific AI as fixed length using a tilde character: '~':
b.Number = "(999~)123456";