-- e-mail like, appropriate for Corporations (Enterprise): if your Company is divided in different departments or countries, you can have a Jabber server for each of those (like e-mail - xxx@yyy.zzz.com aaa@mmmm.zzz.com); the users of these servers see each other through s2s connection.
-- ISP like environment: you have one domain and want to scale the number of users on that domain.
the approach here is that of server farming, which TIMP does nicely - each server can have around 1,000 or more concurrent users (due to windows limitation on sockets), and you can farm together the servers. From our experience, in an ISP environment, you have a ratio of less than 1/10 (concurrent users/ registered users), which means that each server can manage 10,000 or more registered users!