Question-and-Answer Resource for the Building Energy Modeling Community
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Infiltration is outdoor air UNintentionally entering the building through cracks, leaks, etc. while the building envelope is "sealed" (doors closed, windows closed, etc.). The window opening events would be an example of natural ventilation -- outdoor air intentionally entering the building for some purpose (free cooling, e.g.). You can read this post for more details on modeling natural ventilation in OpenStudio.
The two approaches of assigning infiltration from your screenshots (Space Type input vs. Space input) are essentially the same, but the space type approach will likely be easier and faster. You can read this post for more information on that topic. This post also discusses using blower door results at 50 Pa and converting that to a natural infiltration rate.
