Article 21 addresses income of a type not otherwise expressly dealt with under the treaty (i.e. interest income is dealt with but gambling winnings are not), and income from sources not expressly mentioned (i.e. amounts paid by third states.) Under the OECD Model the exclusive right to tax such income belongs to the state of residence unless the income is associated with a PE that a resident of one contracting state has in the other contracting state.
None of the NAFTA treaties follow the OECD Model in this regard. The Canada-U.S. Treaty (Article XXII) provides that "items of income of a resident of a Contracting State, wherever arising, not dealt with in other Articles shall be taxable only in the state of residence, except where the income arises in the other Contracting State it may be taxed in that other State." The Canada-Mexico Treaty (Article 20), US-Mexico Treaty (Article 23) and the UN Treaty (Article 21) contain similar provisions.
Under the Canada-US Treaty, and the Canada-Mexico Treaty if income falls
within article XXII, the rate of withholding tax on income distributed
from an estate or trust that is resident in Canada to a US resident, who
is the beneficial owner of the income, is limited to 15 per cent of the
gross amount of the income.