Google Shopping (and other CSEs) require identification information for products so they can group similar products together. There are typically 3 distinct fields: GTIN (Global Trade Item Number), MPN (manufacturer part number), and the brand. We'll detail all 3 of these below and also discuss how you can utilize Google to find this information for some products.
Google publicly displays these 3 product attributes for some items, and it can be extremely helpful to match what other retailers are utilizing for similar products. Do this by following the steps below:
You will need to find a "grouped" product listing to see product identification information (look for "Compare prices" or "from XXX stores").
If you see "Compare prices" or "from XXX stores" for your item, click it and scroll down to the "Details" section near the bottom.
Once you find the "Details" section, you should be able to find the "Brand", "Part Number", and "GTIN" or "UPC". You may need to click "View all details" to see this information.
Some products will have multiple part numbers and/or GTINs. For these products, you will need to choose only one brand, part number, and GTIN. Do not specify multiple identification information for your products.
If you do not see grouped listings, you can try making your search more general. If you still don't see a grouped listing for your product, you will not be able to use Google Shopping to find identifier information.
Always strive for data accuracy, if you see products that are "close" to what you sell, but not the same — use your own identification information.