The Serial Shipping Container Code (SSCC) is a widely accepted numbering scheme to uniquely identify logistic/shipping units such as pallets. An SSCC-18 number consists of 18 digits which are preceded by a 2-digit prefix which is called as the Application Identifier (AI).
In EDI world, SSCC-18 numbers are mentioned in the Advance Shipping Notification (ASN) documents and then the same SSCC-18s are included on the labels of the corresponding shipping containers. On the shipping labels, SSCC-18 numbers are usually included as GS1-128 formatted barcodes, so that they can be easily read using the barcode readers during handling operations.
A SSCC-18 number is a combination of the following 5 segments.
The Application Identifier is the 2-digit prefix of the number. For SSCC-18 numbers, the value of Application Identifier should be always 00.
The next digit is called the Extension Digit, which is generally being used to indicate the packaging type of the shipping container. This value can be any digit between 1-4, but for simplicity you can use 1 as the value of this digit.
The next 7-10 digits contains the GS1 prefix assigned for the company from whom this container is originating. The very 1st digit of that segment is always 0 for North American (US & Canada) companies and can be any non-zero value for others.
Based on the number of products allowed for a company, the length of the GS1 company prefix varies between 6 to 9 digits (excluding the 1st digit mentioned above). So in some cases it is possible that the GS1 prefix occupying less than 10 digits (non or part of the amber colored positions). In such cases, the rest of the digits can be occupied by the serial number.
Serial number is the identifier of this specific shipping container, which can be 6 to 9 digits long based on the length of the GS1 prefix. Basically the total length of GS1 prefix and the serial number should be 16-digits.
Since all the other segments of a SSCC-18 number except for the serial number can be common to more than one shipment container of a company, the serial number is responsible for making the SSCC-18 number unique for this specific container. Therefore the best and simplest approach is to generate sequential serial numbers for your shipments.
Usually Walmart requires their vendors not to repeat each SSCC-18 number within 2 years. Therefore basically you should be able to assign a unique serial number for each of your shipments within a 2-year period.
The last digit which is called the Check Digit is calculated based on the values of the preceding 17 digits excluding the Application Identifier. You can use the Check Digit Calculator provided by GS1.ORG to calculate check digits for your SSCC-18 numbers.
If you are interested in the algorithm used to calculate this check digit, you can read about it from here.