They are different things. The UPN (Unique Pupil Number) is used to identify pupils in state schools. The UCI (Unique candidate identifier) is used to identify students for entries for GCSE and A levels (and some other qualifications). There is also the ULN (Unique learner number) which is obligatory in state schools and used for entries for a wider variety of qualifications. In theory the ULN allows records of all the qualifications you have done to be held in a single electronic place but independent schools don' use them and they haven't replaced the UCI as the main identifier for GCEs and GCSEs.
There is no need for you ever to know your UPN but you need to make sure you retain the same UCI (and ULN if used) even if you change schools.