The other 100,000 letters (CP59 notices) are going to more than individuals with incomes of between $400,000 and $1 million in income between tax years 2017 and 2021. Some non-filers have multiple years included in the case count so the number of individuals who are the subject of this effort is less than the number of letters.
In all cases, the IRS has received third-party information that the individuals had income in the reported ranges, but failed to file returns.
The effort to chase down these taxpayers has been only sporadically since 2016 because of lack of funding. The new campaign was made possible by funds in the Inflation Reduction Act.
Compliance letters have started going out with about 20,000 to 40,000 more to be sent each week, beginning with those in the highest-income categories.