[Dovecot] Master auth: what is 'client-id'?

[Koen Vermeer]

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