Those are encoding rates for conversion to mp3. Higher rates are less compressed and therefore damage the audio less, but they yield larger files. 192 is a pretty good compromise if 320 is too big, which for anybody on dialup, it would be. Really the dialup folks need 128.
It's not just for MP3. It could really be for any kind of encoding format.
For example, 128 AAC sounds at least as good as 256 MP3. (to my ears, anyways)
They say 96 WMA is about the same as 128 MP3.