The Poser manual states:

"You can adjust the cache size by clicking and dragging on the Irradiance caching slider. More caching means that ....  irradience values will be less accurate due to the increased amount of estimation. Less caching .... will result in longer processing times but increased accuracy."

But the render window help prompt says:

"Higher values increase detail in ambient occlusion and gather effects."

If the manual is correct, then zero on the slider gives no caching (= highest accuracy).

If the in-program prompt is correct, then 100 on the slider gives highest accuracy (= no caching). Also, if you copy the highest Automatic render settings into the Manual tab, it gives a value of 100.

Does anyone have a definitive answer on this?

 I would try both. In PRman there is a parameter in the Environmental Light called MaxVariation. Let's say we want ambient occlusion in our scene. A MaxVariation of zero will calculate all the hemispheric sampling of required to created nice smooth ambient occlusion, which of course takes oodles of time to calulate.

As you increase the value of MaxVaration, hemispheric sampling decreases and the renderer interpolates values between the samples based on shading rate. So values of >=10 for draft renders, a value of 1 for most final renders, occasionally lowering MaxVaration towards 0 in the most demanding scenes (for example when many, many objects are very close together).

Many of PRman's coolest effects (including raytracing) can be cached and pre-processed, written into "3d textures" for use in subsequent renders, which I believe is what Firefly does in one step. Like shading rate, a lower value may equal higher accuracy. Try it the first time with a value of one and again with a value 10. Look at the resulting renders and I'm sure you'll be able to see the difference. 

IC = 100 is basically no caching. So higher IC produces more accurate shadows and longer render times.

In the parlance of maximum tolerated error (replicand was talking about this) the error is 1 - IC/100. Thus, when the IC value goes down, the error goes up.

Some threads you may find informative on the subject of Poser AO.

http://www.renderosity.com/mod/forumpro/showthread.php?message_id=3088474

http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2628527

http://www.renderosity.com/mod/forumpro/showthread.php?message_id=3099231

Thank you replicand and bb... not only a definitive answer to my simple Q, but a wealth of background information, and I'm still only on the first thread.

Those two jade dragons are superb. I'm just cleaning the fingerprints off my screen, having tried to pick one of them up.