What is a UCI?

UCI stands for “unique client identifier.” It is also known as a client identification number (client ID). It’s on official documents you get from us. It has either:

• four numbers, a hyphen and four more numbers (Example: 0000-0000) or
• two numbers, a hyphen, four numbers, a hyphen and four more numbers (Example: 00-0000-0000)

If you are applying to us for the first time, you will not yet have a UCI. If 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 type “N/A,” leave the space blank.

Source : http://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=013&top=4