acct_unique

[Velikanov]

 Good day.

I need to use such a construction in radiusd.conf

 acct_unique acct_std{
         key = "User-Name, Acct-Session-Id, NAS-IP-Address,
Client-IP-Address, NAS-Port"
 }
 acct_unique acct_PIX{
         key = "Acct-Session-Id, NAS-IP-Address, Login-IP-Host,
Login-TCP-Port"
}

preacct {
        preprocess
        Acct-Type PIX{
              acct_PIX
                                }
        acct_std
                }

BUT got an error  when i do "radiusd -X":
..........
..........
Module: Instantiated sql (sql)
radiusd.conf[1683] Unknown module rcode 'acct_PIX'.

In  radiusd.conf there are the strings:
string 1682:     Acct-Type PIX{
string 1683:         acct_PIX
string 1684:                                }
WHERE IS MY MISTAKE ?
And a questions - Why we need to use attribute "Acct-Unique-Session-Id" ?
I use sql database for accounting, there we have unique index "radacctid"
and i need
to understand the purpose and advantages of using the attribute
"Acct-Unique-Session-Id"

THANKS