TurboTax generates IA 126 for you, based on your answers in the Iowa interview. To get the IA 126, you have to be a nonresident or a part-year resident.
The purpose of the form is to determine revenue as Iowa-sourced, even when a non-resident. At the start of the Iowa interview, you are asked if you want to change your residency for Iowa, based on your federal return (most people don't need to). Once it is determined that you are a nonresident or part-year resident, then TurboTax asks a series of questions to determine if certain income items are Iowa-sourced income (and therefore taxable in Iowa).
If you are an online TurboTax customer, you will be able to see your state forms after you have paid. Even if you e-file, you should save a copy of your federal and state returns as a PDF file.