This field will be removed from the schema
in a future release. Recommended usage as of release 439 varies for
eBay.com listings and Half.com listings.
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For eBay.com listings
As of release 439, this field can still be passed in, but we recommend
that you update your applications to use the ExternalProductID field
defined on the item instead (i.e., Item.ExternalProductID).
If you specify both Item.ExternalProductID and this field in the same
request, eBay uses the value in Item.ExternalProductID and ignores the
value in this field. See Item.ExternalProductID for information on using
an external ID for eBay.com listings.
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For Half.com listings
As of release 439, this field is required for Half.com listings.
Causes Half.com to list the item with Pre-filled Item Information based
on an ISBN value or other supported external ID, plus other meta-data
that you specify. See the eBay Web Services guide for information about
listing to Half.com.