That digit is the check digit. That check digit is usually added to the end of the already encoded numbers. The computer checks that the numbers were read correctly by comparing the check digit it calculates against the check digit it read. Reading a check digit of 392 and calculating a check digit of 392 from the other numbers it read, the bar code reader now is doubly certain that the numbers were correctly decoded. Bar code technology is millions of times more accurate than typing when it comes to entering information into the computer.
Check digits make the systems even more accurate.
Tests have shown that operators may do 10,000,000 entries between errors when using check digits. How small the bar code can be becomes a function of how finely the printer can print and how fine of a bar code a reader can read. Industry guidelines will strongly caution against undersizing bar codes. This is good advice. There may be times when you want bar code as small as possible, however, this is only true until it becomes less readable.