The mules as referred to by Debbie were used to get the assembly lines built and prove out all the fixtures and jigs made to produce the production cars. I used to build panels for mules that were actually carbon fiber layups from CNC milled Ren plank models. These were than assembled using the test jigs and an entire vehicle would be assembled. They were also used to see if any assembly operations would be effected so earlier changes could be made if necessary.
the carbon fiber panels were most often made available before any serious cutting was done on the Stamping dies.
Mules would then be made from kirksite dies stamped panels were assembled again and fine tuned the line using these mules (generation II) and then final mules using cast iron stamping dies were built and sent down the line to test. These vehicles were more costly to build and could not be sold since they cost more to build. GM could write off the entire value by crushing them rather than selling them for less and not being able to put the cost toward the development. It makes entire economic sense when you understand that!