the existence of this property is anti-thetical to the idea that build directories are opaque.
all user-visible execution should happen from an install directory only.
this ensures that there is only one way to specify which files end up where.
if no top-level install directory is specified, it is set to a well-known (and configurable) subdirectory of the build directory, and installation becomes an automatic step of the build. as possible optimization, qbs could shortcut the install step by building final artifacts directly into the install dir - however, i would recommend against that, as it would mean additional complexity in the Rules. instead, installation should be optimized by use of hard links.