July,

"Multi-zone" is a type of AHU. It does not mean that there are multiple
AHUs serving the same space. A multizone air handler separates the air
in the unit and blows some air across a cooling coil and the rest of the
air blows across a heating coil. Dampers at the end of the AHU then mix
some of the hot air with some of the cold air to get the mixed air
temperature needed to meet the load in the space for each of the zones.
If the air handlers are like this, then your air system selection is
correct. I would not expect a multi-zone air handler to be used in a big
open space. They are used where there are different thermal zones that
need to be served by the same system. I suspect that the air handlers in
your building are constant volume, single zone, instead of multi-zone.
Hope this helps.

Sincerely,

Keith Swartz, PE, LEED(r) AP