I'm simplifying here, but..

All lights have a shadow cam, even IBL. If you enabled depth mapped shadows on a light it gets used. It uses the shadow camera to render what the light "sees". Why? Because if the light can see a particular spot, then that spot can see the light, which means it is lit by the light. Paint it white. If the light can't see a spot, it is in shadow for this light - paint it black. So it sort of makes a black-and-white render from the shadow camera, painting all the objects black or white and saves this infomation. (This is why you can reuse them - the information doesn't need changing unless you move an object or light.)

Soft shadows are made by blurring this hidden shadow camera render. Sort of.

Then later, as it scans the scene for your render with your camera, it consults that shadow cam render for every spot to find out if it was also drawn there and how bright. If it wasn't drawn in there, it assumes it was lit. If it was, it looks at the brightness to determine how much to light it.

I wonder if a missing shadow cam might come back if you enable depth-mapped shadows on the light and render, but I'm just guessing.

I suppose Poser could barf, but light doesn't need it's shadowcam unless you enable DM shadows on the light and render with shadows enabled.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)