Usually, the system aperture is the first parameter defined when starting a new design. The system aperture not only defines the size of the beam which ZEMAX will trace through the optical system, but it also determines the initial direction cosines of the rays launched from each field point in the Object plane.
Entrance Pupil Diameter is perhaps the most commonly used system aperture type. In ZEMAX, the EPD is defined as the diameter of the pupil in lens units as seen from object space. That is, axial rays at the object appears to enter the system entrance pupil.