As you know, it is an unique client identification number. To find this number, depends on your situation. If you are already inside Canada applying for sponsorship, you already have an UCI number. It is on your visitor record or work visa record, when you came into Canada. However if you are applying outland in your own country, then you won't have an UCI assigned and CIC will assign one to you when they start processing your application. So you leave the UCI field blank.
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