Is access available to my customers/suppliers outside of my Local Area Network(LAN), Wide Area Network(WAN) or Intranet?

Yes. Since the communications medium is the Internet. As long as your users have an Internet connection and a browser (e.g. Chrome, Firefox, Safari etc), they have everything they require.

What parts of the tracking system, and how much access they have within each part of the system, is controlled on a per user basis by passwords and privileges granted by you.