Google - what info Google wants for ALL products

Basically, Google wants the following information on all products.

BRAND
MANUFACTURER-PART-NUMBER

Google calls this combination of data the Unique Product Identifier

For some products, Google requires an actual UPC code or an ISBN number instead of just a Manufacturer-Part-Number.  Google calls these extra numbers GTIN.  If you are a merchant and you sell products where BRAND and Manufacturer-part-number info is normally available, you must include it.  If you sell CUSTOM MADE or  one-of-a-kind items where no manufacturer part number or brand even exists, then you must use the "identifier exists" field and mark it FALSE.