the sponsor will receive their UCI number when they receive sponsor approval letter or if CIC contacts them regarding missing paperwork (by email or mail). it's the number you and CIC will use to identify you in their system for any immigration needs going forward. you'll also be able to use this number to sign on to the online system, ecas. besides that, there is no other use for the number.
A UCI doesn't suggest approval. the letter stating the decision is what suggest approval or rejection. a UCI is created for anyone who applies for anything immigration related. people who are approved and people who are rejected all receive a UCI. it is a tracking number for applicants. that is all.
the applicant may receive their UCI if their visa office sends out an acknowledgement of receipt (AOR) once the file has arrived at the visa office. most offices do not send this so a lot of applicants do not receive their UCI until CIC contacts them regarding an application request, passport request or approval.