Three problems, each solved by a different piece of paper.
Take a UK artist offered a 12-date run across the US East Coast. Three months of preparation, a tour manager, a lighting rig, two bandmates, and £80,000 of hired and owned . The routing is locked. Then the questions start.
The first is the right to be there: a foreign performer needs a work visa to enter the US and play commercially. The second is the right to bring the gear: backline and production kit cross borders as a temporary import, and without the right document each piece can attract import duty and , that's the carnet. The third is keeping the fee: when a US pays a foreign artist, the US can tax that income at source before it ever leaves the country, and a tax treaty plus the right forms is what reduces that bite. Three documents stand between a profitable tour and a very expensive disaster.