Any investor who conducts the entire investment transaction (from introduction to execution of documents) outside the U.S. at the time of application may be considered a Reg S Investor. Any investor inside the U.S. for any part of the transaction must be a Reg D Investor pursuant to U.S. securities laws.
A Reg D investor must be an accredited investor and there are several ways of accreditation including net worth, income, or a certification from a professional such as an attorney, financial adviser, or accountant.