Unique client identifier. You will get that number only once and then you will use it for any application for all type of visa including PR when dealing with Canada.
Once you are UCI identified you cannot do much flip - flops in your EE profile.
That would also explain all those scheduled maintenance. The only question is if first compare new accounts with existing ones or if they just add new UCI to them and only compare applicants against their UCI pool (to see if there is no duplicity).
And what would somebody do if ever he or she was registered under 2 different accounts (not profiles, as you might need to have more than one if the old would expire for one reason or the other).