eBay's unique identifier for a specific version of a product. See Pre-filling Item Specifics with Product Details
for information about finding this type of product ID. (For the shorter product ID (ePID) value that is displayed on the eBay Web site, see ProductReferenceID instead.)
If the primary and secondary categories are both catalog-enabled, this ID should correspond to the primary category (not the secondary category).
In item-listing requests, if you pass in an old product ID, eBay lists the item with the latest version of the product and the latest product ID, and the call returns a warning indicating that the data has changed.
Some sites (such as eBay US, Germany, Austria, and Switzerland) are updating, replacing, deleting, or merging some products (as a result of migrating from one catalog data provider to another). If you specify one of these products in a request, the call may return the product with a warning, or it may fail with an error if the product has been deleted.