How is Form 1042 different from section 1446 partnership withholding and from the Form 5472 baseline?

These are genuinely different systems. The Form 1042 and 1042-S regime, driven by Chapters 3 and 4, covers US-source FDAP and FATCA withholding on payments to foreign persons. Section 1446(a) governs withholding on effectively connected taxable income allocable to foreign partners, and section 1446(f) governs withholding on certain transfers of partnership interests. Form 5472 is a baseline information report for related-party transactions of a reporting corporation, including a foreign-owned US disregarded entity treated as a corporation for that limited purpose. The IRS treats the FDAP/FATCA system and the partnership-withholding system as distinct, and the 5472 baseline is distinct from both.

A foreign-owned LLC taxed as a partnership can have section 1446 obligations, Chapter 3 obligations, or both, depending on whether a given item is ECI allocable to a foreign partner or US-source FDAP. And the 5472 plus pro forma 1120 baseline does not replace Forms 1042 and 1042-S when withholding or reporting on US-source payments to foreign persons is otherwise triggered. The classic mistake is assuming the 5472 you already file somehow covers your FDAP withholding — it does not. Our /guides/section-1446 walkthrough draws the line between the partnership regime and the FDAP regime.

There is a single targeted overlap worth knowing. If a partnership withholds on a foreign partner's share after March 15 of the following year and designates the deposit as attributable to the prior year, it reports that income and tax on the prior year's Forms 1042 and 1042-S, and the due date for those 1042-S forms becomes September 15 in that limited situation. That is a timing rule inside the reporting system; it does not merge section 1446 into Chapter 3 or vice versa. See /guides/form-1042-s for how the calendars interact.