The Z/ZX -120V version is intended to operate at 120VAC single-phase. The 240V version is intended to operate at 240VAC three-phase. The only difference between the -120V version and the -240V version each uses a different type of fan optimized for its own voltage range. This means that a -240V version operated at 120VAC will dissipate less heat away from the drive (the fan rotates slower). A -120V version at 240VAC will damage the fans.
The difference in performance between the two is that the available speed from a -120V version will be approximately half that of the -240V version.