What is the difference between standard density flow rate and current density flow rate?

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The node volume flow rate can be viewed as mass flow rate using standard density (20 C and 0.0 humidity ratio) or actual density (using actual temp and humidity ratio). Each of these evaluated at the current barometric pressure (weather file altitude).

state.dataEnvrn->StdBaroPress = DataEnvironment::StdPressureSeaLevel * std::pow(1.0 - 2.25577e-05 * state.dataEnvrn->Elevation, 5.2559);
state.dataEnvrn->StdRhoAir = Psychrometrics::PsyRhoAirFnPbTdbW(
            state, state.dataEnvrn->StdBaroPress, DataPrecisionGlobals::constant_twenty, DataPrecisionGlobals::constant_zero);

or

this->stdRhoAir = Psychrometrics::PsyRhoAirFnPbTdbW(state, barometricPressure, 20.0, 0.0);

versus actual air density calculated as:

Real64 rhoair = Psychrometrics::PsyRhoAirFnPbTdbW(state, state.dataEnvrn->StdBaroPress, ActualTemp, ActualHumRat);

There are several component models in EnergyPlus that convert volumetric air flow rate to mass flow rate using actual air density (e.g., DX coils) and others that convert volume air flow rate to mass using standard air density. Depending on which model is used, these report variables are available when post processing calculations are desired. If post processing is not desired, then these report variables are not all that useful except to see the component flow rate throughout the simulation and that the flow rate is what is expected.

Output:Variable,*,System Node Mass Flow Rate, hourly; !- HVAC Average [kg/s]
Output:Variable,*,System Node Standard Density Volume Flow Rate, hourly; !- HVAC Average [m3/s]
Output:Variable,*,System Node Current Density Volume Flow Rate, hourly; !- HVAC Average [m3/s]
Output:Variable,*,System Node Current Density, hourly; !- HVAC Average [kg/m3]