Check out this thread.
http://www.canadavisa.com/canada-immigration-discussion-board/client-id-and-uci-t65715.0.html
http://www.canadavisa.com/canada-immigration-discussion-board/what-is-uciclient-id-t76213.0.html
Or this from the CIC website.
http://www.cic.gc.ca/english/information/applications/guides/5256E4.asp
If this is your first time dealing with Canada immigration, you probably dont have one and should leave it blank.
Thanks mmadsen55 for your answer.Iwant to apply for my mother.As my sister from Canada have already applied for my parents(for permanent visa).Is that file number is UCI or not?My mom is applying first time visitor visa.What should i write on that box? pls help me.