When header notification is enabled, the header and first few lines of the message will be printed on your terminal whenever mail arrives. A "biff y" command is often included in the file .login or .profile to be executed at each login.
When bell notification is enabled, only two bell characters ( ASCII \007) will be printed on your terminal whenever mail arrives.
If no arguments are given, biff displays the present notification status of the terminal to the standard output.
The biff utility operates asynchronously. For synchronous notification use the MAIL variable of sh(1) or the mail variable of csh(1).