ISSN (International Standard Serial Number) is complementary book numbering system to ISBN. An ISSN symbol consists of the letters ISSN followed by ten (10) digits which are: a group indentifier (1-6) digits) and a check digit (1 digit). The group identifier identifies the nation, geographic or other similar grouping of publishers. The publisher indentifier indentifies a particular publisher in a group which is assigned by Serials Data System. The title identifier identifies a particular title or edition of a title publised by a particular publisher. The check digit is calculated from the preceeding digits. Hyphens are used to separate the groups of digits. The ISSN code are printed in machine-readable OCR-A characters.

When you select ISSN barcode symbology you will have to type 9 digits with 2 hyphens to yield a complete barcode. For example: inputting "0-724581-91-X", would generate the following code (see below).

The barcode symbols that appear on magazines are encoded using EAN-13 barcode. However, although the type of bar code is the same as others used for retailers, the numbering system used to generate the bar code is different. The EAN for normal retail products is a 13 digit number which uniquely identifies that product. However, a serial publication like a magazine already has a unique number to identify it, the ISSN. The EAN bar code for a serial publication is generated from the ISSN for the publication.

When encoded in an EAN-13 bar code, the ISSN is preceded by the digits 977 and the ISSN check-digit (the last of the 8 digits) is not encoded. A two digit price code, almost always "00", is added to the end, and finially the EAN check digit is added.

Let's say you have the number 0264-3596 as a serial publication's ISSN. The true ISSN is 0264359 (no 6 because it is the ISSN check digit). The EAN-13 number would then be 9770264359008 for normal issues. The "00" is the normal issue code and the "8" is the EAN-13 check digit). A special issue could receive a 2-digit numbers other than "00".

Some serial publications also have an add-on 2-digit code that represents the issue number.

You can find much more information about how to apply for an ISSN code and other information at ISSN.ORG.
There is more information about ISSN at the United States Library of Congress.