There are exactly two grounds, and the statute states them narrowly. Section 6654(e)(3)(A) allows the IRS to waive the addition "to the extent the Secretary determines that by reason of casualty, disaster, or other unusual circumstances the imposition of such addition to tax would be against equity and good conscience." Section 6654(e)(3)(B) covers a taxpayer who "retired after having attained age 62, or became disabled, in the taxable year for which estimated payments were required to be made or in the taxable year preceding such taxable year," where the underpayment "was due to reasonable cause and not to willful neglect."
Two details in that second limb are easy to lose. It is a two-year window keyed to the event, not a status: the retirement or the onset of disability has to fall in the tax year itself or the year immediately before it, so being retired for a decade is not a ground. And reasonable cause is a separate requirement stacked on top of the event, which means the retirement has to explain the shortfall rather than merely coincide with it.
The asymmetry between the two boxes is the most useful mechanical fact here, and it runs against intuition. Form 2210's Part II offers box A, "You request a waiver (see instructions) of your entire penalty," and box B, "You request a waiver (see instructions) of part of your penalty." Asking for all of it is the lighter task. The instructions say that a taxpayer checking box A completes only page 1 of the form and attaches it to the return, and "you aren't required to figure the amount of penalty to be waived." A taxpayer checking box B must complete the form through line 18 without regard to the waiver, enter the amount they want waived in parentheses on the dotted line next to line 19, subtract it from the penalty figured without the waiver, and enter the result. Partial relief therefore costs a full computation that total relief does not.
The written statement is not optional and its contents are specified. The instructions require the taxpayer to "attach Form 2210 and a statement to your return explaining the reasons you were unable to meet the estimated tax requirements and the time period for which you are requesting a waiver." Naming the period matters, because the charge is computed installment by installment, so a circumstance that existed in September has nothing to say about an April installment. A request grounded in retirement or disability must be supported by documentation showing the retirement date and the taxpayer's age on it, or the date disability began. A request grounded in a casualty or other unusual circumstance calls for documentation such as copies of police and insurance company reports.
Filing the form is what makes the request, and that reverses the normal posture. The default under the Form 2210 instructions is that "the IRS will generally figure your penalty for you and you should not file Form 2210." Requesting a waiver is one of the situations the instructions name in which a taxpayer must file it. Nothing on the return itself signals a waiver request, so a taxpayer who simply pays the bill when it arrives has not asked for anything.
Federally declared disasters mostly run on a separate track. The IRS identifies taxpayers in a covered disaster area by county or parish while processing the return and applies penalty relief automatically, and the instructions say that in general a taxpayer whose underpayment was due to a federally declared disaster should not file Form 2210 at all. The exception is worth knowing because it reaches people who assume they are outside the relief: an individual or fiduciary not in the covered area but whose "books, records, or tax professionals' offices are in a covered area" is also entitled, as are relief workers affiliated with a recognized government or charitable organization assisting in the relief activities. Someone in either of those positions is directed to call the IRS disaster hotline and identify themselves as eligible, rather than to rely on the automatic identification.