Why in Natural Ventilation is there "fan efficiency" ?

Hi there! I'm in OpenStudio -> Thermal Zones -> Zone Ventilation Design Flow Rate -> Natural Ventilation. I'm mooving this natural ventilation to the "zone equipment" of each thermal zone. Now, for this natural ventilation I'm setting data like schedules and air change per hour. My qyestion is: why are there in this section data like "fan pressure rise", "fan total efficiency"? I know that natural ventilation is just something like opening windows, why are there data to fullfill about fans? thank you

In the Ventilation Type field if you set it to Exhaust or Intake, the assumption is that there is a fan drivingthe flow which requires you to enter the Pressure Rise and Fan Efficiency. Balanced is a combination of Intake and Exhaust so you also need to enter those two fields for that selection too. If you set it to Natural there is not assumed to be a fan and those fields are optional.

Source

As an aside, the majority of questions related to objects and inter-workings can be answered by looking at the EnergyPlus Input Output Reference or Engineering Reference, always check there first.