The DO-254 guidelines apply to civil aviation for aircraft, helicopters and engines as mandated by the Federal Aviation Regulations. The guidelines apply to the systems and equipment that utilize software or airborne electronic hardware used in aviation.
The FAA indicates that avionics equipment consists of both hardware and software that are critical to airborne safety. RTCA/DO-254 is the Design Assurance Guidance for Airborne Electronic Hardware. Complex electronic hardware that fall under DO-254 consist of Field Programmable Gate Arrays (FPGAs), Programmable Logic Devices (PLDs), and Application Specific Integrated Circuits (ASICs).
Yes. It would be difficult to prove that functions of different Design Assurance Levels did not interact with each other. Shared clocks and routing resources also introduce coupling and dependencies that the designer did not intend. Credit for floor planning features in PLD design tools should not be assumed.