Bar Width Reduction (BWR)
Bar Width Reduction (also referred to as Bar Width Adjustment) is used to compensate for printing processes that output bars wider or narrower than desired. Commonly, commercial printing equipment will introduce a certain amount of ink spread (a.k.a. press gain) that you should compensate for, since a barcode is made of precise measurements.
The adjustment is specified in milliinches (mills). You can make the adjustment in the "Export Options" window on the barcode page. This value should be calculated based on the dots-per-inch (dpi) of your printer - that is, the number of dots on a page when printing at a specified resolution. For example, a 600 dpi printer prints 600 dots for every inch. So, an adjustment of "-1 dots" for 600 dpi resolution will result in a bar with reduction of 1/600th of an inch. We use the adjustment in mills, not dots, as it is dependent on the resolution of the output device.
The amount of adjustment needed is different for different types of output devices. Your commercial printer or prepress manager can provide more information on ink spread, and will likely know the term "Bar Width Reduction" and the specification you need while exporting your barcode.
Users set this option to achieve technical precision when using certain kinds of printing equipment – if you're using this option, spend the time to fully understand the printing process and find the correct value.