Since many products are similar and have similar names, it's important to include a unique product identifier. There are several options available to indicate product identifiers in the schema.org vocabulary.

Schema.org offers several properties to indicate a product's GTIN. GTIN stands for Global Trade Item Number, a standard created by GS1, and it is the most widely used product identifier globally. Many products will have a GTIN, which will range in length between 8 and 14 digits. If you know the GTIN for the product, it's recommended that you include it, because it is widely used. Choose the property that matches the length of the GTIN for the product:

• gtin14 - is 14 digits, and is the globally recognized standard for identifying products everywhere in the world.
• gtin13 - is 13 digits, and is used in Europe and some other regions where it is known as the European Article Number (EAN), and used in Japan where it is known as the Japanese Article Number (JAN). It is also used as a 13 digit product identifier for books, which is known as the International Standard Book Number (ISBN). Schema.org offers a special ISBN property to indicate the product ID for a book.
• gtin12 - is 12 digits, and is used in North America where it is known as the Universal Product Code (UPC).
• gtin8 - is 8 digits, and is used for small products such as cosmetics that can't accommodate a larger label.

You can also indicate the a manufacturer's or vendor's unique ID using these properties:

• mpn The Manufacturer Part Number (MPN) of the product, which can be alphanumeric digits of various lengths.
• sku The Stock Keeping Unit (SKU), which may be used for the vendor's unique ID for a product, which which can also be alphanumeric digits of various lengths.

It's possible for a product to have several unique identifiers. The product may be identified by an identifier other than GTIN, MPN, or SKU. You can indicate these other kinds of IDs using the productID property.

Some other common product IDs you can indicate using the productID property are:

• ASIN (Amazon Standard Identification Number) 10 letters and/or numbers
• GPI (Generic Product Identifier) for therapeutic drugs, a 14-character ID
• NSN (NATO Stock Number, or National Stock Number as it is known in the US), is a 13-digit numeric code, known as the DMC (Domestic Management Code) in the UK.

Publishers have the option to include any kind of product ID when using the productID property.

When using the productID, indicate the ID system first, follow with a colon, then indicate the numeric or alphanumeric ID. You can list several IDs together using the productID by using an array. For example:

{
  "@context": "http://schema.org",
  "@type": "Product",
  "productID":  ["nsn:123456","asin:991133","barney:RugsackCanvasBlack123"],
  "description": "U.S. Army official canvas rugsack (takes a licking keeps on kicking)",
  "name": "Rugsack, canvas, black"
}