HDR is one of several formats that are used in HDRI. The codings can be used with a lightprobe node directly. The default images supplied for IBL use (Which is the correct term for what Poser is using, a subset of HDRI) are in jpg format, and really are little more than combined photogrpahs of reflective spheres -- the core source used for HDRI, which the software may or may not be converting natively. The red bounce is due to gather.