To use it, type an 11- or 12-digit number in the "Code" field. If you type 11 digits, the 12th digit (the check digit) will automatically be populated in the generated bar code image for you. If you type 12 digits, the check digit will be validated against the correct check digit. A validity indicator is visible to the right of the "Code" field.
You can set the horizontal scaling factor (a percentage value), to values between 80 and 200 percent, and the vertical bar size (in millimeters) to a value between 10 and 100 mm. Option-clicking the size sliders will cause them to return to their default values.
You can also set the font for the generated bar code. The default font is Helvetica 10. You can option-click the font button to return it to its default value.
You can save MakeUPC documents for reuse; the saved format is a native application format. You can also "Save As PDF…" to save a PDF file containing the generated UPC-A bar code graphic, or you can drag from the bar code view in the application to the desktop or another application that accepts dropped PDF data.