[Exim] Defining my own director

Hello,

at the moment I am writing my own director for delivering mails via a
commandline tool. Now I have some questions:

If I define my own director like this:

imap_delivery:
  driver                = pipe
  command               = "/usr/local/cyrus/bin/deliver ${localpart}"
  return_path_add
  return_output

What do I specify via driver? According to the documentation the following:
"This option must always be set. It specifies which of the available
directors or routers is to be used."

Is pipe now a director or a router?
Where is it defined or is it predefined?
Where is this driver explained?


Thanks a lot in advance!!!

Bye

Oliver