The prefix used in the built package.
Each CPack generator has a default value (like /usr). This default value may be overwritten from the CMakeLists.txt or the cpack(1) command line by setting an alternative value. Example:
set(CPACK_PACKAGING_INSTALL_PREFIX "/opt")
This is not the same purpose as CMAKE_INSTALL_PREFIX which is used when installing from the build tree without building a package.