This refers to the percentage of air, from the total air supplied to the room, that is being circulated back. So, this air is coming from your room itself, and not from outdoors. Most ventilation systems recirculate a portion of the room air to save on cooling/heating costs. The fraction of recirculation may typically vary between 70 and 90 percent. This should be provided to you by your building services engineer/building management office. We use this value, in conjunction with total air supplied to the room, to estimate how much outdoor air - air without virus laden particles - is being supplied to your room.