assiwassi said:
Universal Client Identification number (UCI) is needed to fill if you have. If you don't have this one, leave it blank. Please read the instruction guide, at first, before filling up your application forms. Hope it will help you a lot.
I
ndicate your Universal Client Identification number (UCI) or Client Identification number (Client ID), if known (8-digit number). Otherwise, leave it blank. If this is your first application with CIC you will not have a UCI or a Client ID.
Source: http://www.cic.gc.ca/english/information/applications/guides/EG7TOC.asp#eg74
Read also the following thread:
http://www.canadavisa.com/canada-immigration-discussion-board/uci-urgent-please/-t102694.0.html