The term "Registered" usually refers to having registered with the national body representing the engineers of that country. Not all degree qualified engineers bother to register. You also usually need 3-4 years experience to quallify, but eligibility requirements will differ. For example in South Africa, you need to write an application stating not only how many years, but also in which roles. More than one role is advantageous (e.g. sales, project management, design, quality assurance, etc.) and applications are considered on a case by case basis.
What it means to the Americans is anybody's guess. Maybe one of them can inlighten us. (P.S. as I've posted several times before...international registered LEED ft² is greater than 27% of ALL LEED ft². I tend to feel the international responsibility from America in general is lacking.)