As of February 19th, 2021, Product Builder is unavailable for new installations. If you are currently using Product Builder, you will be able to continue using the app as your account will now be grandfathered.
The final Product Builder product can represent one, or many, SKUs in your store. You will need to make sure that the product(s) you connect to Product Builder have a SKU within the Shopify admin.
Ultimately, the SKU of your Product Builder is entirely dependent on which product(s) you have within your Product Builder's Option Select steps.
You sell tablets with 2 priced options: iPad and Case. Each have a SKU. When a customer builds their tablet and selects both iPad and Case, the final Product Builder product will be represented by 2 SKUs: 1 for iPad and 1 for Case.
Alternatively, you sell tablets with only 1 priced option: iPad. When a customer builds their tablet and selects iPad, the final Product Builder product will be represented by 1 SKU: iPad.