If you purchase charts for the system, all MFDs on that network will share the same licence and be able to show the chart. (As long as they have the chart available on the chart chip) If you have a mixed system and have an established SYSTEM ID that already has other map licences, then you should use that system ID when you order. If you have never purchased charts, NOW is the time to determine which unit you want to be your "Chart Master". Normally that is the unit you would power on first or if you came to the boat and only powered on ONE unit, which would it be? The chart master should normally be the one which gets used the most on the boat. Once licences are cut they belong to that unit. If the chart master MFD is sold, so goes the older charts purchased for it. That being the case, you might consider having one of your TZT3 units be the chart master if you don't already have one firmly established. When TZT2 and TZT3 are are on the same mixed network, the chart master will work for both systems and use the same shared SYSTEM ID.