CAST-15, Merging High-Level and Low-Level Requirements is a Certification Authorities Software Team (CAST) Position Paper.

It is an FAA publication that "does not constitute official policy or guidance from any of the authorities", but is provided to applicants for software and hardware certification for educational and informational purposes only.

As established by the FAA advisory circular AC 20-115, the RTCA publication DO-178B/C defines an acceptable means of certification of airworthy software.

[5] This position paper is also an example of Certification Authorities using their "what" versus "how" distinction[6] between high-level and low-level requirements[7][8][9] that DO-178B/C does not clearly explain.

This is also an issue for any usage of development tools that potentially reduce the resolution of requirements in a project, particularly those that use notations of higher abstraction (e.g., UML or block flow modeling).

This CAST-15 Position Paper is no longer provided on the FAA's publications site as the team's concerns were addressed by FAQ #81 in DO-248C, Supporting Information for DO-178C and DO-278A[12] and by changes and clarification in the release of DO-178 Revision C: However, neither DO-248C nor DO-178C completely incorporates the "full discussion of this topic"[5][7] that is recorded CAST-15.

FAQ #81 also recommends against merging High-Level and Low-Levels even in cases where the code can be written and verified in a "single step" of requirements as the original DO-178B/C guidance allows, but does offer suggestions on how to address concerns.