per diem amounts
(1) Luxury water transportation (A) In general No deduction shall be allowed under this chapter for expenses incurred for transportation by water to the extent such expenses exceed twice the aggregate per diem amounts for days of such transportation. For purposes of the preceding sentence, the term “per diem amounts” means the highest amount generally allowable with respect to a day to employees of the executive branch of the Federal Government for per diem while away from home but serving in the United States. (B) Exceptions Subparagraph (A) shall not apply to— (i) any expense allocable to a convention, seminar, or other meeting which is held on any cruise ship, and (ii) any expense described in paragraph (2), (3), (4), (7), (8), or (9) of subsection (e).
26 USC § 274(m)(1)
None identified, default scope is assumed to be the parent (part IX) of this section.