SKU (stock keeping unit) is an identification code, usually alphanumeric, of a product that allows it to be tracked for inventory purposes.
IngramSpark can assign a free SKU to your book for printing purposes if you do not intend to distribute your book through Ingram; just select “Click here if you would like a non-distributable SKU to be assigned to this title”, during the title set up process.
If you should decide to add an ISBN and barcode to the book at a later time, you will need to set up the same title as a new title and either purchase an ISBN from IngramSpark for $85 or supply an ISBN during the title setup process. You will also need to upload the interior and cover files. A barcode is furnished at no cost.