IDL in poser is light bouncing. its a way to simulate how lighting works in real life. light bounces around in the real world. plus IDL in poser is also able to use every texture and geometry as a lightsource. so if we have a box in our scene and the ambient is set to white then it will light our scene.

if people dont understand this with 100 articles on google then they shouldnt use it. i know i sound mean but  there is enough info on this forum and on google. i doesnt get any easier then this. its  light bouncing aroud. if we have at home a white wall then lets put our hand next to the wall. we will see a very small orange tint. its light bouncing from our skin on the wall.

IDL means indirect lighting.