HDRI makes a program work like Vue, or Bryce, where the outer limits of the "World" have color/light values that always reflect inwards, and in a number of directions.  This way reflective metal materials actually work.  By default the outer limits of the Poser Universe have no light, and are just Duds...really.

Another term for this would be Sky Dome.  In the past month or so I've done some considerable work with reflections/sky domes.

What you can attempt to do is add lights to your scene from behind/through where you light would be coming from in your 2D Panoramic HDRI image.  Light is then filtered through the image/Sky Dome and very realistic lighting is present when compositing a render of actual photography.