If we're talking about that very small group, basically, we're talking about less than 0.3% of tax filers, just to emphasize how small that group is. They are going to be very much one-time cases for those individuals in the large majority of cases. It could be things like a large medical expense.
Another example might be someone who's been in school and has earned tuition tax credits, but often while you're in school you don't have enough taxable income to use them. You can carry them forward. Maybe you've accumulated enough when you get your first job and you have enough taxable income and then you can bring in these very large non-refundable tax credits.
I think that example is a good illustration of how temporary—how one-time—that effect can be for, again, an extremely small number of people. Going forward, as that individual continues to work, they would benefit from the tax reduction.