Another special case of synchronization is a one way sync. During a one way sync the actual sync data is only sent in one direction. This could be used for a public server that gives out information, like the “Drehscheibe”, which is a university calendar. Any student could connect to it and download her schedule, but not change anything.
Even a sending only client seems possible. Some E-mail programs, for example, have the capability to automatically add every person you have sent mail to to your address book. This would be a one way sync requesting the addition of an entry, if not already present.