HDR stands for High Data Rate or something similar. At some point back in time Media 100 only supported 4:1 compression (150/180k NTSC/PAL), and to get the best result with this compression you were able to choose between different compression algorythms. I think with V2.5 or V3 Media 100 introduced HDR and compression up to 2:1 (300/360k). This only works with the HDR setting, all the other compression settings still went only up to 150k, although the data rate could be set higher. So HDR was the recommended format since at least V3, and with the switch to V8 and OSX the “old” settings are only there for legacy reasons. There is no reason why you should not work in HDR; in fact you will get a much better image quality.