Every cross-border tax question eventually collapses into the same underlying issue: is this dollar of income U.S.-source or foreign-source? It sounds like a formality, but the answer drives some of the most consequential numbers on an international return — the foreign tax credit limitation under IRC §904, whether a nonresident alien owes U.S. tax at all under §871 or §882, which income falls into which FTC “basket,” and how a tax treaty's sourcing tie-breakers apply.