For NSC folks it is the LIN number, for VSC folks enter the EAC number.
The first meesage that you will hear is "No Record of approval yet". About 2-4 weeks from I-140 approval (although NVC claims 6-8 weeks), this message will change to "NVC has received the approval and case number number is xxxxxxxxxxxxx ". This is a 13 digit number. From here on you can either use your LIN/ EAC number or NVC generated case number for follow up.
In my case, I-140 approved on 4/1/03.
Message changed at NVC on 4/10/03 to NVC assigned case number BMB2003xxxxxx.
On 4/16/03 message changed to "NVC received DS3032".
On 4/21/03 message changed to "NVC issued fee bill"
My attorneys are still waiting to receive the fee bill, after which they will make payment to St Louis, which will encash payment, inform NVC which will then issue Packet 3 to attorneys which you will fill and mail to NVC which in turn will send to Consulate in India, which in turn will mail you packet 4 which is nothing but your invitation for the interview. For approx. time frames refer cptracker.com
hope this clarifies!