Good Day ALMALL,
I like your question and this will help those who will read your question and my comment. Here we go, when someone applied for TRV in the past CIC issued a UCI # that goes together with that visa application. Remember that an application number can be destroyed after sometime but not UCI#. Let's give an example here to clarify what I am talking about. If you applied for a TRV 15 years ago and went back to your country, then on the 16th year you decide to apply for PR to come to live in Canada for good; your PR will have the same UCI# you had 15 years ago because UCI # is like your digital finger print. No one will ever be given those numbers, no one has ever had those numbers except you and you only. Same as your finger prints: God never created anyone with the same finger print you have and He will never created anyone with the same finger prints even if you have passed on. So that was a beat explanation of UCI #.