A lot of client meetings still start with the same assumptions: W-2 in a U.S. state, mortgage nearby, a 401(k), maybe some RSUs, and a college-savings plan. Yet many of those “simple” clients now own an apartment in another country, freelance for a foreign company, or have parents wiring money from overseas. The tax code does not see those details as background color; it treats them as organizing facts. This is where citizenship, residency, and domicile quietly step onto center stage. For planners, the challenge is not memorizing every cross-border rule. It is knowing when a client is no longer “just domestic” and shifting your planning framework before you stumble into penalties, double taxation, or blown opportunities.