26 U.S. Code § 6603 - Deposits made to suspend running of interest on potential underpayments, etc.
A taxpayer may make a cash deposit with the Secretary which may be used by the Secretary to pay any tax imposed under subtitle A or B or chapter 41, 42, 43, or 44 which has not been assessed at the time of the deposit. Such a deposit shall be made in such manner as the Secretary shall prescribe.
To the extent that such deposit is used by the Secretary to pay tax, for purposes of section 6601 (relating to interest on underpayments), the tax shall be treated as paid when the deposit is made.
Except in a case where the Secretary determines that collection of tax is in jeopardy, the Secretary shall return to the taxpayer any amount of the deposit (to the extent not used for a payment of tax) which the taxpayer requests in writing.
For purposes of section 6611 (relating to interest on overpayments), except as provided in paragraph (4), a deposit which is returned to a taxpayer shall be treated as a payment of tax for any period to the extent (and only to the extent) attributable to a disputable tax for such period. Under regulations prescribed by the Secretary, rules similar to the rules of section 6611(b)(2) shall apply.
For purposes of this section, the term “disputable tax” means the amount of tax specified at the time of the deposit as the taxpayer’s reasonable estimate of the maximum amount of any tax attributable to disputable items.
Except as otherwise provided by the taxpayer, deposits shall be treated as used for the payment of tax in the order deposited.
Written determinations for this section
These documents, sometimes referred to as "Private Letter Rulings", are taken from the IRS Written Determinations page; the IRS also publishes a fuller explanation of what they are and what they mean. The collection is updated (at our end) daily. It appears that the IRS updates their listing every Friday.
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