That is what the IRS (and most tax offices in the world, such as the British HMRC or the Italian AdE) defines as "foreign source income" since the work is liter

That is what the IRS (and most tax offices in the world, such as the British HMRC or the Italian AdE) defines as "foreign source income" since the work is literally being carried out abroad. It also includes capital gains and interests arising abroad of course.