This category is used for listing MP3 Directory sites. These are sites on the net that list other MP3 sites/software/skins in categories usually with a search function so you can find just the right site you're looking for. You can usually add your site to a directory by going through a procedure explained there, or if you have an MP3 Directory site, you can list it here by going back to the category page and clicking "add URL".
If your directory has sites/software etc. separated into categories, then it can be listed here. Otherwise if its just a bunch of links all on one page, it should be submitted to Arts/Music/Sound_Files/MP3/Link_Lists/.

Please also note that this category DOES NOT list sites offering the download of MP3 files (i.e. download directories). These sort of sites should be submitted to Sound_Files/MP3/Downloads/.