The variable SZA defines the Solar Zenith Angle (in [deg]), which varies between 0 deg (at zenith) to 90 deg (over the horizon), with a default value of 55 deg.
This variable controls the strength of the solar irradiance (weighted by cos(SZA)). Also, it defines the length and geometry of the sun-target optical path, which increases/decreases the effects of absorption and scattering caused by gasses and aerosols.