Somebody will correct me if I'm wrong, but the ATPL (frozen) is what is needed to become an airline pilot. This includes both the CPL (commercial pilots licence) and IR (intrument rating). If you want to fly for any airlines, this is needed.
The CPL is required if you want to earn money from your flying, which could include flag flying and the likes(?). Or pleasure flights.
Don't know if anyome can answer this question - do you need a CPL to be able to get the FI rating? And to become an instructor is the class 1 med. required? Just curious!
JW.