Manufacturer can tell you specific overall pressure and also external static pressure (the one that you can use outside the unit after all AHU internals are already accounted for.
If you stell manufacturer you need 2" ESP, they will give you an AHU with 3.5" total Static pressure and 2" ESP. Meaning the internals use 1.5". for design ESP is relevant. total pressure really is only important for fan sizing, energy etc.
some manufacturers give you smaller AHU, that have more overall pressuredrop. then they give you tiny high-speed fans. that one will be much more expensive to operate than larger unit with larger fan. so compare properly.
