Under the Product schema type, there are multiple ways of identifying a product:gtin12gtin13gtin14gtin8mpnSo why does the productID property exist? Isn't this a little redundant?
What's the difference between productID and gtin/mpn (http://schema.org/Product)
schema.org
GTIN 8, GTIN-12, GTIN-13, GTIN-14 and MPN are all common global and local identifiers which replace a wide range of previous popular identifiers.SOURCEGTIN-14 Former name(s): DUN-14, ITFGTIN-13 Former name(s): EANUCC-13, EAN-13, CIPGTIN-12 Former name(s): EANUCC-12, UCC-12, UPCGTIN-8 Former name(s): EANUCC-8, EAN-8It's worth mentioning that ISBN was superseded by EAN, and EAN was superseded by GTIN-13. On the 1st of January 2005 the U.S. ISBN agency requires publishers be able to communicate ISBNs as GTIN-13. However, Schema allows the use of product ISBN for older books, you could use productID, or ISBN, both are correct. ProductID is an less specific product identifier, you could use GTIN, MPM or another identifier that is not supported by GTIN, such as 'Product ID BRAND'. You will find that with Schema there is often times that you will find markup that can be approached in different ways, however the outcome remains the same, and correct.