The Content tab of the Barcode Screen allows you to specify the exact data your barcode should bear. Depending on the barcode type, the exact amount of data may vary. For example, the EAN-8 barcode type holds up to 7 symbols of numeric data plus one CRC number.
To enter the barcode data simply type them into the appropriate fields. Press the Tab button to reflect changes on the preview.
Barcode automatically detects a number of typical errors in the barcode data. Whenever such a problem is detected, a warning message is shown at the bottom of the preview window. Click the Fix Errors button to resolve such issues as insufficient data, wrong CRC numbers and other errors. However, please note that the program uses default values (zeroes) to fill in insufficient data and mercilessly cuts off excessive symbols when needed. Just keep that in mind when planning your barcode contents.
ISBN barcodes have specific content properties, see here for more details.