It is domestic. At least, if you connect through SJU (San Juan, Puerto Rico). Caribbean Sun serves both STT and STX from SJU. They take jump seaters with no problem, back and forth. Flying directly to the US Virgin Islands from the mainland USA, I had a gate agent try to collect taxes from me. The USVI are considered USA, but you have to clear customs. Same is true when you travel from one posession to another (like from SJU to STT). Hope that helps. JS to and from Puerto Rico is pure domestic.