For every market, a single market reference which will uniquely identify the <ProductSupply> composite which describes the market within this Product record, and which will remain as its permanent identifier every time you send an update.

The Market reference is optional and non-repeating, but it is strongly recommended unless there is only a single market. It is intended to be used to label each repeat of <ProductSupply> for use in subsequent partial updates and reporting.

Note that the scope of the Market reference is limited to a single Product record – a geographically identical market for a different product may have a different Market reference, and a geographically different market in another Product record may use the same reference.