dovecot - Apr 2007 - Master auth: what is 'client-id'?

Hi,

I am trying to get some useful data from the master auth socket. On the
wiki, I found that I need to do:
M: "REQUEST" TAB <id> TAB <client-pid> TAB
<client-id>
M: "USER" TAB <id> TAB <userid> TAB
service=<service> [TAB <parameters>]

However, what should I specify for <client-id>? I cannot find a
definition on the Wiki.

Thanks!

Koen