Merging High-Level and Low-Level Requirements

Section 5.0 of DO-178B/ED-12B explains that when Source Code is generated directly from high-level requirements (HLR), then the high-level requirements are also considered lowlevel requirements (LLR), and the guidelines for low-level requirements also apply. This part of section 5.0 addresses the case where the system-level requirements are "directly" highly refined (i.e., created in one refinement step); hence, allowing a single level of software requirements. However, applicants sometimes misuse this paragraph to justify combining high-level requirements (HLR) and low-level requirements (LLR) into the same data item. They establish development cycles that are intended to expand HLR with requirements built during the software design phase. Thus, they are “flattening” the requirements tree and merging all the requirements into a single data item, without establishing traceability between LLR and HLR. In general, HLR represent “what” is to be designed and LLR represent “how” to carryout the design. Merging these two levels leads to several certification concerns, as described in this paper.