Ventilation Zones are specific to the Ventilation Calculations and on the platform are defined as each room. This provides maximum flexibility to define varied occupancy categories, ventilation effectiveness's etc.
The requirements for Ventilation Zone are described in the ASHRAE Standard 62.1 User Manual. The key factors are:
Similar Occupancy Category
Similar Occupant Density
Similar zone air distribution effectiveness
Similar primary airflow per area (similar thermal loads)
While similar is not a defined term, 10% difference is a common rule used when considering two values similar.
Why are Ventilation Zones different from HVAC Zones?
A HVAC or thermal zone is defined as containing a single thermostat. While many HVAC zones also meet the requirements describe above, this will not always be the case. For example, consider an Office and Corridor on the same HVAC Zone. While they both can be control by the same thermostat, the occupant density, and thermal loads will vary more than our 10% rule. This means they would not be considered the same Ventilation Zone.