Value engineering (VE) is the disciplined search for cheaper ways to achieve the same performance — swapping a specified system, detail, or material for a less expensive one that still meets the project's needs. It's most powerful early, in preconstruction, when changes are cheap.
Done well, VE trims cost without gutting quality (a different roof membrane with the same warranty and R-value). Done poorly, it's just cost-cutting that comes back as callbacks and owner frustration. The best VE ideas often come from the subs and superintendents who build the work daily.