[alsa-devel] Some questions on snd_kontrol_new

Could someone get me some insight into the snd_kontrol_new? I'm especially interested in the "private_value" and "index" field. At least in usb_mixer.c it seems to me like private_value is used as the index!? If that's the case, when should index be used and when private_value? Is there any special convention for the string of "name" field?