Fee Receipt: There is a fillable area and one being "Client ID". I wasn't sure so found this on CIC site:
What is my client ID?
Your client ID, also referred to as a UCI, appears on all documents you get from Citizenship and Immigration Canada (CIC). It is an eight-digit number that looks like this: 0000-0000.
If you are applying to CIC for the first time, you will not yet have a client ID. 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=012&t=4