871(m) - a history of complexity. If you trade in derivatives or equity linked instruments (ELIs) with any kind of underlying US exposure and, as a result you make dividend equivalent payments (DEPs), you will enter the murky world of US Section 871(m) regulations. But don't worry, its easy to navigate (NOT).
There are complex rules about what role each financial institution in the payment chain will play, what their obligations are, what tests they need to perform to determine withholdability and reportability and while the IRS kicks the can down the street, your ability to rely on a deferral is contingent on the obligations to meet (and prove that you meet) 'good faith efforts' standards.