This refers to where the aperture of the spectrometer is oriented with respect to the plasma flame. Radial view has the spectrometer oriented perpendicular to the axis of the flame and axial view has the spectrometer oriented parallel (as though the flame is pointed right at you). As I recall axial has significantly better sensitivity (because the effective path length is longer) but also more chemical interferences (because you are also assaying light emitted from the cooler part of the flame, where atomization is less efficient). A dual instrument allows you to select which mode you use for any given experiment.