If you don't specify a product_code, it will be generated from the product_name column. If the product_name is not specified it will be generated from the first image or the first file of the images and files column. If based on that the system is not able to find a product_code, the product will be skipped. If a product_code already exists in the database it will be skipped.
Finally, you can also insert variants of products. You will have to specify in the product_parent_id column the product_code of the main product which must be inserted before the variants in the file. You will also have to have column names matching the characteristics in the store.
For example, suppose that you have a product with 1 characteristic being the size and two variants:
product_code,product_parent_id,size
main_product,,big
first_variant,main_product,big