In the Advanced tab of any mailbox properties, you can find two fields named ILS Server and ILS Account. What are those fields for? The help section in X5.5 says something about Netmeeting, mailbox owner finding and Microsoft Internet Locator Server.
You're on the right track! Microsoft Internet Locator Server is an LDAP-based directory service for Microsoft NetMeeting. An ILS maintains a directory of NetMeeting users, enabling them to select participants for real-time conferencing and collaboration.
If you've ever installed NetMeeting on a machine running Outlook, you may have noticed that NetMeeting integrates itself into Outlook, enabling you to invite someone from an Outlook address list to a NetMeeting. If you populate an Outlook user's Exchange mailbox properties with the ILS Server and ILS Account info, Outlook will be able to pass the information to NetMeeting, which will then be able to do an LDAP lookup against the ILS server's in-memory database of NetMeeting users, locate the invitee, and start the conference.
If you're running NetMeeting and wanting to further deploy or leverage conferencing in your organization, then look into Exchange 2000 Conferencing Server. For more information on the ILS, see https://www.microsoft.com/en-us/windows and http://www.netmeet.net/.