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Leveling to areas with different heights

This is something I've noticed a few times. On a print, -A- datum will be called out as 3-4 areas of different heights. (Say, areas 1 & 2 are 133mm in the Z, area 3 is 136mm in the Z, and area 4 is 128mm in the Z) How exactly does this create a level plane? How should I go about my alignment?