Parent comment really gives a great overview of the process. One slight nitpick is that DO-178B or DO-178C are revisions of the standard and the associated cri

Parent comment really gives a great overview of the process. One slight nitpick is that DO-178B or DO-178C are revisions of the standard and the associated criticality is associated with the Design Assurance Level (DAL) from A to E. One would say software is developed to DO-178C DAL B for example. The DAL is determined before the software is started at the Systems level according to a process described in ARP4754A (Guidelines For Development Of Civil Aircraft and Systems) looking at the system architecture, hazards, and potential mitigations of those hazards, one of which is developing software to a certain DAL.