26 CFR § 301.6229(c)(2)-1 - Substantial omission of income.
(a) Partnership return -
(1) General rule.
(i) If any partnership omits from the gross income stated in its return an amount properly includible therein and that amount is described in clause (i) of section 6501(e)(1)(A), subsection (a) of section 6229 shall be applied by substituting “6 years” for “3 years.”
(ii) For purposes of paragraph (a)(1)(i) of this section, the term gross income, as it relates to a trade or business, means the total of the amounts received or accrued from the sale of goods or services, to the extent required to be shown on the return, without reduction for the cost of those goods or services.
(iii) For purposes of paragraph (a)(1)(i) of this section, the term gross income, as it relates to any income other than from the sale of goods or services in a trade or business, has the same meaning as provided under section 61(a), and includes the total of the amounts received or accrued, to the extent required to be shown on the return. In the case of amounts received or accrued that relate to the disposition of property, and except as provided in paragraph (a)(1)(ii) of this section, gross income means the excess of the amount realized from the disposition of the property over the unrecovered cost or other basis of the property. Consequently, except as provided in paragraph (a)(1)(ii) of this section, an understated amount of gross income resulting from an overstatement of unrecovered cost or other basis constitutes an omission from gross income for purposes of section 6229(c)(2).
(iv) An amount shall not be considered as omitted from gross income if information sufficient to apprise the Commissioner of the nature and amount of the item is disclosed in the return, including any schedule or statement attached to the return.