The calling machine on a PPP link is considered the authenticatee because the caller must prove its identity to the remote peer. The peer is considered the authenticator. The authenticator looks up the caller's identity in the appropriate PPP files for the security protocol and authenticates or does not authenticate the caller.
You typically configure PPP authentication for a dial-up link. When the call begins, the dial-out machine is the authenticatee. The dial-in server is the authenticator. The server has a database in the form of a secrets file. This file lists all users who are granted permission to set up a PPP link to the server. Think of these users as trusted callers.
Some dial-out machines require remote peers to provide authentication information when responding to the dial-out machine's call. Then their roles are reversed: the remote peer becomes the authenticatee and the dial-out machine the authenticator.
PPP 4.0 does not prevent authentication by leased-line peers, but authentication is not often used in leased-line links. The nature of leased-line contracts usually means that both participants on the ends of the line are known to each other. Both participants often are trusted. However, because PPP authentication is not that difficult to administer, you should seriously consider implementing authentication for leased lines.