Question about pam.d

[Paul E Condon <pecondon@mesanetworks.net>]

Last night a new version of qpopper for squeeze appeared in my 
aptitude display and I downloaded and installed it. The only 
change that I could see was that the authentication settings
have been changed. I'm looking at what was done in order to
better understand how Debian/Gnu/Linux works. The new 
/etc/pam.d/qpopper contains:

#%PAM-1.0
# /etc/pam.d/qpopper - specify the PAM behaviour of qpopper
#

# We fall back to the system default in /etc/pam.d/common-*

@include common-auth
@include common-account

# We don't use password or session modules at all
# @include common-password
# @include common-session

For my situation, bypassing authentication is absolutely the best choice,
but I am curious as to how these lines achieve that goal.
I look at the man page for pam.d and find an explanation of the
format of this file. BUT... there is no mention of a service type
'@include'. 'include' is explained, but not '@include'. Explanation? 

-- 
Paul E Condon           
pecondon@mesanetworks.net