The article discusses how different countries tax income earned internationally. It explains that countries have rules to decide who gets to tax this income. The researchers argue that while there are clear rules for where income is earned, there are not clear rules for how to divide this income between countries. This lack of clear rules can lead to tax planning strategies that exploit loopholes in the tax system. It is important to have fair and clear rules to protect the income tax base of the United States and other developed countries.