A product identifier is how you, and the marketplaces you sell on, identify a product. There are multiple product identifiers, not all are needed for all marketplaces, and not all can be used for all marketplaces. This is a comprehensive list of what the product identifiers are and any specifics that need to be kept in mind when creating them!
SKU- A Stock Keeping Unit (or SKU) is an identifier that is unique to you and your business. This is one that you will create yourself, and use to keep track of your items in Listing Mirror and your marketplaces. Listing Mirror does require that your SKUs are unique to each product.
ASIN- Amazon Serial Identification Number (or ASIN) is a unique alphanumeric identifier provided by Amazon itself. Every listing on Amazon has a unique ASIN. Listing Mirror will use this ASIN to link multiple offers you have on a single item.
UPC- Universal Product Code (or UPC) is a 12 digit product identifier. It is typically found underneath the barcode on an item's packaging, and is used primarily in North America. This product identifier is required for Walmart listings (Unless you use a GTIN) but not required on other marketplaces.
EAN- European Article Number (or EAN) is most similarly compared to a UPC. It is a 13 digit product identifier. Like the UPC it is typically found under the barcode on an item's packaging, however unlike the UPC, it is typically used in Europe.
GTIN- Global Trade Item Number (or GTIN) is a 14 digit product identifier that can be used by any business to identify a businesses trade items. A trade item is defined as a product or service that has been priced, ordered, or invoiced at any point in it's supply chain. This is an acceptable replacement for Walmart's UPC requirement.
ISBN- International Standard Book Numbers (or ISBN) is the product identifier used for books. The ISBN was only 10 digits up until the end of 2006, but in 2007 and on they are now 13 digits. This is important to remember in case you run into any ISBNs that are not 13 digits. The ISBN was founded in the 70's so if you are selling books published before then and are not a reprint, they may not have an ISBN.
MPN- Manufacturers Part Number (or MPN) is a unique product identifier that is typically created by the manufacturer and used when ordering items directly from them. eBay does require an MPN for items in specific categories (like automobile parts), however as long as the MPN is unique to the item in your account, you can use any product identifier you have for the item in it's place, or create your own unique Product Identifier!