see if you can solve your own question:So in some states having a license to be a physician is the same thing as being a certified medical practitioner? If you are a certified, trained doctor, why would you need a license to practice medicine?
licensed/certified mean the same thing.
you have to have at least a journeyman's license before you can break out on your own. most states require that you register as a licensed plumber though. I know here in FL you have to. they are very strict about it. in fact they are over the top about it. if you get caught doing work without being registered you can be fined and kicked off the job site.