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sizeSystem

A Schema.org Property

This term is proposed for full integration into Schema.org, pending implementation feedback and adoption from applications and websites.
The size system used to identify a product's size. Typically either a standard (for example, "GS1" or "ISO-EN13402"), country code (for example "US" or "JP"), or a measuring system (for example "Metric" or "Imperial").
Values expected to be one of these types
SizeSystemEnumeration
Text
Used on these types
SizeSpecification

Source

https://github.com/schemaorg/schemaorg/issues/2811