Positive and negative pressure this two term you might find in lots of engineering applications in mechanical & electrical engineering. Like a transformer room, switchgear room, gas processing unit needed to be maintained in either negative or positive pressure as it requires.
For both the negative and positive pressure, the room must have to be air tight.
What is positive pressure
Positive pressure is used where outer air is not required in the room. Like in operation theater of hospital surroundings air might carry air-borne germs. Or in some production areas where outer air might deviate the quality of product like in pharmaceutical industries.
What is Negative pressure
In simple word a room will be in negative pressure if the air pressure in the room is lower than that of the surroundings air pressure. So no air flow from the room under negative pressure. If any leakage exist, air will ingress (flow inward).