Sometimes, there is no internal wall between two zones, but just a opening. So, how to describe this are in SU with legacy open studio. Though I can draw this situation in SU software, I'm not sure whether this method accord with the principle of software
If they are two different spaces that you plan to put on two different thermal zones you can use an air wall construction for the opening and add a ZoneCrossMixing object for each thermal zone.
If they are two different spaces that you plan to put on the same thermal zone, the opening is extraneous.
For Legacy specific answer just skip the part about the air wall construction. That is unique to the OpenStudio Model format, and only affects Radiance, not EnergyPlus. Also the ZoneCrossMixing won't be exposed in the legacy plugin, so you will need to add that to the IDF file externally.
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