Initially released in 2000, the DO-254/ED-80 standard was not necessarily considered policy until recognized by the FAA in 2005 through AC 20-152 as a means of compliance for the design assurance of electronic hardware in airborne systems.
An item is simple "if a comprehensive combination of deterministic tests and analyses appropriate to the design assurance level can ensure correct functional performance under all foreseeable operating conditions with no anomalous behavior."
Conversely, a complex item is one that cannot have correct functional performance ensured by tests and analyses alone; so, assurance must be accomplished by additional means.
With DO-254/ED-80, the certification authorities have indicated that avionics equipment contains both hardware and software, and each is critical to safe operation of aircraft.
At this point the PHAC (Plan for H/W Aspects of Certification) is presented to the authorities (EASA, FAA...).
Methods of verification include qualitative review, quantitative analysis, and functional testing.
A Plan for Hardware Aspects of Certification (PHAC) should be submitted to communicate the justification and means of certification, and a Hardware Verification Plan should be submitted to communicate the rigor and methods of the deterministic testing and analysis.