Brownfield vs greenfield
Greenfield work starts from nothing, no legacy code, data, or constraints, while brownfield work modifies an existing system with its accumulated history, integrations, and technical debt. The distinction sets expectations: greenfield optimises for speed and clean design; brownfield optimises for safe incremental change, because the cost lives in not breaking what already works.
The distinction sets honest expectations. Greenfield work, a clean start, is seductive but rarer than people wish; brownfield work, changing a living system with existing data, integrations, and users, is most real engineering, and it's slower for legitimate reasons: every change must respect constraints the original authors are no longer around to explain. Pretending a brownfield project is greenfield is a reliable way to underestimate it by half.