Due to the simulation-based nature of Iray Photoreal, its behaviour is different to that of the other rendering modes in many aspects. For example most scene options which control global illumination and other advanced lighting aspects do not apply to Iray Photoreal.
Iray Photoreal does not support classic rendering features such as ambient occlusion, final gather or photon mapping but catches global illumination effects automatically without the need of explicitly controlling different stages of the rendering process and thus avoids tweaking various scene dependent parameters.
Due to this "push-button" simulation approach, it is not possible to exclude objects from casting or receiving global illumination, shadows, etc. The same is true for backface culling. Those flags will silently be ignored. It is also not possible to switch certain global illumination features off as these will always be included. The only exceptions are the effect of the iray_max_path_length rendering option that can for example be used to just get direct lighting contribution and the primary visibility flag for scene objects. In addition, the concept of light path expressions was created to enable very similar workflows and effects instead, but in a much more general fashion that in addition is still physically plausible.