dovecot - Oct 2013 - understanding user_attrs and mail_uid/gid

Hello,

I'm trying to understand what's the difference between those parameters.
In my dovecot.conf in the global section I have a definition of mail_uid
and mail_gid.
In my LDAP configuration used by passdb and userdb, I have a definition for
user_attrs= uidNumber=500,gidNumber=8.

Here is a part of my configuration files (dovecot version: 2.1.7)

dovecot.conf


mail_gid = 8
mail_uid = 500


passdb {
  args = /etc/dovecot/dovecot-ldap.conf
  driver = ldap
}


userdb {
  args = /etc/dovecot/dovecot-ldap.conf
  driver = ldap
}


dovecot-ldap.conf

user_filter = (&(objectClass=posixAccount)(mailLocalAddress=%n))
user_attrs = uidNumber=500,gidNumber=8


So everything works fine when all those parameters are present in my
configuration files. If I suppress one of them nothing is working (mail
reading or delivering).

Can someone tell me where I can find a definition of those arguments, if
found nothing relevant on the wiki.

Thanks,