I don't understand what your problem is. WHich operating system do you use?

If you're refering to iTunes sharing of music you can find explanations under your iTunes Help menu. Quote: If your computer is connected to any other computers over a local network (LAN) and you're using iTunes 4.5 or later, you can look for music that's being shared by those computers and have it appear in your Source list.
ie: you can use playlists from other computers on that network. This does not apply to sharing across p2p on the Gnutella Network. However, in mac OSX's (not sure about windows version) preferences/options you can enable LW's music to be shared across the LAN in the same way. Search DAAP on the forum.

If you're refering to sharing iTunes' music across the Gnutella network, then it's just like any other file you share. You're simply sharing iTunes music library with other people so they can dwnld their choice of songs. You can choose which of iTunes music you wish to share by just adding those folders of music you'er willing to share. eg: Mr Smith (artist)>no.1 (album name) I hope you make sense of that.