Ну там вроде написано чем отличаются. Дабы не искажать оригинал -
The z21 version for beginners (white box, little z at z21) has the same features as the Z21 for experts with only a few smaller connection and configuration limitations. Left out are LocoNet, CAN-Bus, the X-Bus port, the Sniffer-Bus, the programming end level and the programmable track port as well as the track current regulator.
Просто про упрошенную версию написано на странице про полный Z21.
http://www.z21.eu/en/Start-with-Z21/Experts