Event Item attributes of the Customer Match.

[Proto definition ![GitHub](https://developers.google.com/site-assets/logo-github.svg)](https://github.com/googleapis/googleapis/blob/master/google/ads/googleads/v25/common/offline_user_data.proto)

| Fields ||
|---|---|
| ## `item_id` | `string` Optional. A unique identifier of a product. It can be either the Merchant Center Item ID or GTIN (Global Trade Item Number). |