http://www.cic.gc.ca/english/helpcentre/answer.asp?q=013&t=4
UCI stands for “unique client identifier.” It is also known as a client identification number (client ID). It appears on official documents you get from Citizenship and Immigration Canada (CIC). It has four numbers, a hyphen and four more numbers. (Example: 0000-0000).
If you are applying to CIC for the first time, you will not yet have a UCI. If so, and you are filling out a form that asks for one, write “Not Applicable” or “N/A” in the space provided.
If you are applying online and the form does not allow you to input “N/A,” leave the space blank
http://www.cic.gc.ca/english/helpcentre/answer.asp?q=772&t=4
Where can I find my client ID/UCI?
This eight digit number appears on all documents you get from Citizenship and Immigration Canada (CIC). It looks like this: 0000-0000.
If you applied to CIC before
Look for this number on:
letters you received from CIC,
your immigration document such as a PR card, study or work permit.
If you do not have any of these documents but want to fill out an application form:
you can leave the field for UCI (Client ID) blank.