Complex sentence: MPCs were fully convertible to US dollars upon leaving a designated MPC zone and convertible to local currencies when going on leave (but not vice-versa), and were illegal for unauthorized personnel to possess, thus, in theory, eliminating US dollars from local economies.
Split into simpler sentences: MPCs were fully convertible to US dollars upon leaving a designated MPC zone, and convertible to local currencies when going on leave (but not vice-versa). They were illegal for unauthorized personnel to possess, thus, in theory, eliminating US dollars from local economies.