As Builts are blueprints that documents how a building was built, after it is "complete". They document what seem like silly details at the time: the placement of pipes and ducts in walls, the exact locations of joists, etc.
the Late, Great Professor Niklaus Wirth called it incremental program building, or something like that
As Alexander admonishes us, we must build systems by Piecemeal Growth. Something built by a Master Plan can never fully attain the Quality Withouta Name. Even the blueprints for large, complex buildings cannot anticipate most design problems. That means that blueprints can't be used as maintenance documentation for the building. An As Built serves that purpose.
-- Jim Coplien (related by Ken Rehor)
See original on c2.com 