This form allows you to specify how to use the leftovers of previous nests. The Theoretical Offcut represents the material that has not had parts assigned to it during a previous nesting process, and has not yet been cut, but the material is sufficient to apply further parts to it from a different nesting process.
Within this screen on the Autocut Settings form, you can:
§ Take the offcut of a previous nest and arrange items on it
§ Disregard the old nest layout that was generated during a previous nest and perform the process again, inclusive of the new parts.