MSCI - Net total return indices reinvest dividends after the deduction of withholding taxes, using (for international indices) a tax rate applicable to non-resident institutional investors who do not benefit from double taxation treaties. (us.spdrs.com)
For example: In Europe, MSCI indexes are often NR, taking as basis the lowest denominator, meaning the country with the worst net of dividend double taxation treaties. (mindfullyinvesting.com)
Typically under double taxation treaties you would see something where if you pay tax in one country, it's subtracted from the tax you have to pay in the other. (money.stackexchange.com)