GI = global illumination.  some of them may provide other definitions, hence it's best to adhere to that given by the guy who wrote the firefly render engine used by poser, who said that IDL is part of GI.  advanced topic. we expect certain improvements as FFRender continues to be developed.

there are poser dome/hemisphere/sphere props in the poser default runtime, but bill's is mapped to take equirectangular panoramas and is distributed with certain advanced shader features.  the poser default "ball" props are mapped wrong IIRC.  don't use them.

HDRI is a 32-bit image, as mentioned above, hence its range of white to black contains much more lighting data than a typical 8-bit jpeg, where black is (00,00,00) and white is (FF,FF,FF).  e.g. 2^8 = 256 (FF + 01), whilst 2^32= FF FF FF FF + 01.