Puerto Rico is a U.S. territory, and its residents are citizens of the United States and have a U.S. passport.
However, persons who were born in Puerto Rico, or whose parents were born in Puerto Rico, are eligible for citizenship in Spain. Clarification: Puerto Rican ancestry accelerates the process of obtaining citizenship in Spain – you still must qualify for emigration to Spain (e.g. work, assets) – but then need live in Spain for only 2 years (compared to 5 years for everyone else) to qualify for citizenship. Puerto Rican heritage does not, by itself, guarantee citizenship – it just accelerates the process.
Yet another group within the U.S. who has unique emigration privileges that are not available to most people.