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asin

A Schema.org Property

This term is proposed for full integration into Schema.org, pending implementation feedback and adoption from applications and websites.
An Amazon Standard Identification Number (ASIN) is a 10-character alphanumeric unique identifier assigned by Amazon.com and its partners for product identification within the Amazon organization (summary from Wikipedia's article).

Note also that this is a definition for how to include ASINs in Schema.org data, and not a definition of ASINs in general - see documentation from Amazon for authoritative details. ASINs are most commonly encoded as text strings, but the [asin] property supports URL/URI as potential values too.
Values expected to be one of these types
Text
URL
Used on these types
Demand
Offer
Product

Source

https://github.com/schemaorg/schemaorg/issues/2288