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I just down loaded a few hdri images. to use with baggins bill env sphere.
when I unpacked them they had a series of different extensions on them. the files were for a milkway system.

3 jpegs and the following
milkyway.ibl (1k)
milkway_light.hdr (67k)
milkyway_small.hdr (6.5M)

Curious as to what the differences are with these and to use them properly with the sphere.

The milkyway_small.hdr is just a photograph of the galaxy. You can stick it on your envsphere and have your reflective objects reflect stars and such.
The milkyway_light.hdr is to be used for lighting (not for reflection). It is just a smaller version of the milkyway_small with a lot of blur in it. You can use it for IBL lights in poser. Because this is used only for diffuse light, the image does not need to have a high resolution, the diffuse lighting calculation munges any details anyway. You could make this yourself by applying a gaussian blur (radius 200 or so) to the milkyway_small and then resizing it to the smaller size.
The milkyway.ibl is just a file describing the different images for a plugin they are distributing on the sibl site (not for poser, though), so you do not need to remember what to do with each image.

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This is from the sIBL site, right?

While that can be used (as in better than nothing) it was made rather poorly. The HDR has no values above LDR maximum, which means it isn't HDR at all. The person who made it thinks that calling a file HDR makes it high dynamic range by virtue of the name and format alone. It doesn't. HDR files that have no values above 1.0 are not "high" - they are just files that could have been "high".

This is not 100% correct. The important thing is the contrast of the image, not the level. The milkyway_small has pixel values between 0.0003 and 1.8 (found out by just picking some dark and bright pixels with the mouse) which gives a dynamic range of at least 1.8/0.0003 which is approximately a contrast of 6000:1. You cannot get this with a jpg image; but i have no idea if this value is correct for the real milkyway.