In GS1 Parlance, this is called Restricted Circulation Numbers in section 184.108.40.206 of the specification.
Restricted Circulation Numbers are used for special applications in restricted environments. They are allocated by GS1 for internal use:
■ RCN-12 is a 12-digit Restricted Circulation Number.
■ RCN-13 is a 13-digit Restricted Circulation Number.
■ RCN-8 is an 8-digit Restricted Circulation Number.
The RCN-8 are 8 digit barcodes with prefix 0 or 2. Many barcode scanner systems and applications have trouble reading these properly. Especially if you attempt to use UPC-E barcodes in the same location.
The RCN-12 have 12 digit barcodes with prefix 04 is for use inside a company and 02 is for use inside a company within a geographic region.
Finally with the RCN-13 you have 13 digit barcodes with prefix 04 and 02 identical to the RCN-12 designations.
Keep in mind you have the prefix, some number of digits, and the checksum. How you arrange the digits in the middle are left up the the site using the restricted circulation number.