I checked on the website, using our UCI (i thought it was the number of Canadian Citizenship), and it was there were i found that it should be an 8 digit number (different) documents. i supouse that UCI code is unique, so it is the code you stated on the form IMM1344.
i am sending you the link. Client Application Status
https://services3.cic.gc.ca/ecas/?app=ecas&lang=en
you press Continue, agree with the conditions and agreements and it will appear the page where you can put your code, and so on.
I hope it is useful the information, to you.
Good Luck.