Typically when you sell a product on Amazon, it comes with a UPC, which is a standard global tracking product code. But since you're selling custom products, they don't have traditional UPCs and need to be tracked on Amazon in another way. This is where unique identifiers come in.
During the brand registry process, you'll be asked to select a unique identifier for each product category you plan on selling.
These are all of the categories on Amazon that apply to products you can sell through Printful and the unique identifiers you need to set for each one:
If you don't select these exact unique identifiers for categories you're selling, then adding products through Printful's push generator won't work. You'll get an error message that looks like this: