Here is your answer:
"The only difference between static pressure and environmental pressure depends if there is vortex
flow near the opening. If there is no vortex flow then there should be no difference.
If there is a vortex near the opening, the flow that is coming back through the opening is calculated
using static pressure if a static pressure boundary condition is applied at the opening. If
environmental pressure is used as the boundary condition then the flow is calculated assuming total
pressure. Environmental pressure is more accurate if there is vortex flow near the opening. If there
is no vortex flow near the opening then the results should be the same no matter if you use static
pressure or environmental pressure."
Static pressure refers to the component of pressure without kinetic energy involvement.
Enviromental Pressure refers to:
- Total pressure (static + dynamic pressure) if the boundary condition is an inlet (e.g.: fluid entering the computational domain). Total pressure = static pressure + 1/2*density*velocity^2.
- Static pressure if the boundary condition is an outlet (e.g.: fluid leaving the computational domain).
Source: SolidWorks Flow Simulation Help
When to use which? Depends on the variables of your problem, if you are going to meassure static pressure or total pressure. If you are ever going to validate this results with experiments, then just see which kind of barometer you've got, if it accounts for the dynamic component or not. Then you'll know your answer.
One quick tip: if you are using Enviroment pressure as an outlet condition, better use static pressure at the inlet to calculate the pressure drop. Makes no sense to calculate static vs total pressure drop.