An extension to file is not an extension to pay tax.
Four situations receive extension treatment outside the standard Form 4868 / Form 7004 framework: U.S. citizens and residents abroad, taxpayers who need more time to qualify for the foreign earned income exclusion, military and support personnel in a combat zone, and taxpayers affected by a federally declared disaster.
A U.S. citizen or resident whose tax home and abode are outside the United States and Puerto Rico on the regular due date, or who is on military or naval duty outside the U.S. and Puerto Rico, receives an automatic two-month extension to file and pay - to June 15 for a calendar-year filer - with no form required. A statement must be attached to the return explaining which situation qualifies the filer. Interest under IRC §6601 still accrues on any unpaid tax from April 15, since this extension only relieves the failure-to-file and failure-to-pay penalties for the two-month period, not the interest charge.
Form 2350 is for U.S. citizens and resident aliens abroad who expect to claim the foreign earned income exclusion or the foreign housing exclusion/deduction (Form 2555) but need more time to meet the bona fide residence test or the physical presence test. It requests an extension beyond the automatic two-month period, to a date after the taxpayer expects to qualify. It is narrower than Form 4868 - available only to filers pursuing FEIE qualification, not as a general-purpose extension.
Under IRC §7508, deadlines for filing returns, paying tax, and other specified acts are postponed for members of the Armed Forces (and certain support personnel) serving in a designated combat zone or contingency operation, plus those hospitalized as a result of such service. The postponement period runs for the time of service in the combat zone (or hospitalization) plus 180 days after the last day in the combat zone, plus any days that remained to take the action when the individual entered the combat zone. No form is required; the IRS identifies combat zone participants primarily through Department of Defense data.
Under IRC §7508A, the IRS may postpone various tax deadlines, including filing and payment deadlines, for taxpayers in an area covered by a FEMA-declared disaster. Relief is announced disaster-by-disaster through IRS news releases specific to the affected localities and dates, and generally applies automatically to taxpayers with an address of record in the disaster area, without a separate request. Because relief periods and covered localities vary by disaster, always confirm current relief at the IRS disaster relief hub before relying on a specific postponed date.