asterisk users - Jan 2010 - What's customer_id mean?

hi ,all

I do'nt know exactly what  customer_id mean?  while  if i have
password i could visit the voicemail box.

CREATE TABLE voicemail_users (
 uniqueid int(11) NOT NULL auto_increment,
 customer_id int(11) NOT NULL default '0',
 context varchar(50) NOT NULL default '',
 mailbox int(5) NOT NULL default '0',
 password varchar(4) NOT NULL default '0',
 fullname varchar(50) NOT NULL default '',
 email varchar(50) NOT NULL default '',
 pager varchar(50) NOT NULL default '',
 stamp timestamp(14) NOT NULL,
 PRIMARY KEY  (uniqueid),
 KEY mailbox_context (mailbox,context)
) TYPE=MyISAM;

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Best regards,
Sucan