Check digit: (Modulo 10) - Automatically calculated by Barcode alpha software.
Use: Retail (consumer goods, books, magazines, newspapers)
Length: 12 digits (11 digits + 1 check digit)
Supported characters: 0 - 9
Class: Numeric-Only Symbology
Special notes:  

UPC-A is a subset of EAN-13. An UPC-A barcode is an EAN-13 barcode with the first EAN-13 number system digit set to "0".
An UPC-A barcode is divided into four areas:
1) the number system,
2) the manufacturer code,
3) the product code, and
4) the check digit.

Number System: The number system is a single digit which identifies the "type" of product, if you will, that the following symbol represents. Normally the number system digit is printed just to the left of the barcode.
Manufacturer Code: The manufacturer code is a unique code assigned by the UCC Council to each manufacturer or company which distributes goods that will include a UPC-A barcode. All products produced by a given company will use the same manufacturer code. Manufacturer Code is printed on the left side just below the barcode.
Product Code: The product code is a unique code assigned by the manufacturer. Product Code is printed on the right side just below the barcode.
The check digit is the last digit and is printed right of the barcode.