Whereas "passive systems" tend to be systems that are integrated into a home when its being built. "Active systems", on the other hand, are systems in homes with a passive system that has been installed on the pipe to "activate it" (place a negative pressure/vacuum on the pipe). Active systems can also be those that need to be "retro-fitted" into the existing space in a home.
Obviously, its more cost efficient to install them when the home is built. But radon sytems can always be installed cost efficiently and with minimal cosmetic impact by experienced professionals to successfully lower the radon levels in a home to acceptable limits.