The 75F Differential Pressure Package is an optional adder to the Outside Air Optimization kit and is used to regulate building pressure. If you have ever noticed your buildings doors are flying open or are sucked shut and difficult to open. I suggest you keep reading.
Even most modern small and midsize RTUs only have a barometric exhaust 'flap' to regulate pressure. Although this 'flap' does allow an excess pressure out of the building, it is only opening to relieve a high excess and has no way to let the unit know the pressure is high or low. This means that the pressure is usually acting on its own which can be hard on the building envelope and can cause irritation when doors are difficult to open, or already open.
By installing a +/-0.25"w.c. pressure transducer to monitor pressure between the outdoors and the interior of the building and feeding that information into the economizer you can now maintain a relatively constant building pressure (assuming the building was reasonable engineered).
It can feel a little complicated if you try to think about the CO2 + Enthalpy + Pressure relationship all at once, so first let's look at two examples that demonstrate how the pressure package would work alone.
Fig. 1 - In the first image lets see that the building pressure is rising above its threshold. In this case what the economizer can do it close to eliminate any excess pressure from building further.
Fig.2 - In this next image you see that the actual differential pressure has fallen to -0.09 which is -0.1 below the setpoint of 0.01inw.c. In this case under default conditions, the OAD will open to 50% to allow pressure to build in the space.
CO2 + Enthalpy + Pressure Relationship
Keep them in that order and it'll be simple, think of them as additive. The CO2 levels are considered the most important since it is a health concern. The CO2 sets the minimum outside air opening. Next, if free cooling is available it will match the load in the space on top of the CO2 minimum. Then, last the pressure will add to the free cooling opening. If free cooling is unavailable, then pressure adds to the CO2 minimum position.