How do I know what I transactions need to be applied for with form 8949?

How do I know what I transactions need to be used on form 8949? I had filed my taxes on TurboTax Deluxe Desktop and it told me I need to send in a form 8453 along with a form 8949. How do I know which of my transactions I need to put on the 8949 page? Shouldn't the software have asked me for this earlier in the filing process?