the reason for the requirement and confusion is essentially building codes and other government regulations: if you want to build or repair a building, you need to hire people to certify the work as meeting building codes; those people need licenses. But at the same time, a company can hire a bunch of fresh engineering graduates to work a such a project and not every one of them needs a license. Unfortunately, we just have the one word engineer that applies to both. "Licensed engineer" would be a clearer distinction than "Professional engineer"
but there has also been recent litigation and a court has ruled that having a degree from an engineering school in an engineering discipline entitles one to use the term https://reason.com/2019/01/02/judge-confirms-that-oregon-eng...
the reason for the requirement and confusion is essentially building codes and other government regulations: if you want to build or repair a building, you need to hire people to certify the work as meeting building codes; those people need licenses. But at the same time, a company can hire a bunch of fresh engineering graduates to work a such a project and not every one of them needs a license. Unfortunately, we just have the one word engineer that applies to both. "Licensed engineer" would be a clearer distinction than "Professional engineer"