Complex sentence: If the user chooses to launch the Classic Environment only when needed, launching a "classic" application first launches Classic, which can be configured to appear in a window resembling the display of a computer booting into Mac OS 9.
Split into simpler sentences: If the user chooses to launch the Classic Environment only when needed, launching a "classic" application first launches The Classic. Environment, which can be configured to appear in a window resembling the display of a computer booting into Mac OS 9.