Represents the sum of payments made with a request for an "automatic" 4-month extension of time in which to file an income tax return, and payments with a request for an additional 2-month extension. Notwithstanding the timing of these requests, full payment of a tax estimate for the year had to be made with the original application; if adjustments to the amount thus paid were later necessitated, the difference was either refundable or payable subject to an interest charge (which is not reflected in the tax statistics).
For additional information about the return filing requirements, see the annual reports, Statistics of Income-Individual Income Tax Returns at https://www.irs.gov/uac/soi-tax-stats-historical-data-tables.
U.S. Department of the Treasury. Internal Revenue Service, Individual Income Tax Filing: Tax Payments: Payments With Requests for Extension of Filing Time [PMTWREXFLA], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/PMTWREXFLA, May 25, 2019.