1446(a): a partnership with ECI must pay tax on behalf of its foreign partners, quarterly, at the highest rate for that partner type — 37% for individuals, 21% for corporations. It doesn't matter whether any cash was distributed. Partners get credit for it via Form 8805.
1446(f): when a foreign partner sells its interest in a partnership that has a US business, the buyer must withhold 10% of the amount realised. Sellers can reduce or eliminate it with certifications — for example, if the partnership certifies that its ECI gain would be under 10% of total gain. If the buyer doesn't withhold, the partnership itself can be on the hook to withhold from future distributions to the buyer — so secondaries teams care a lot.
Worked example
A Japanese pension sells its stake in a US infrastructure fund on the secondary market for $40m. Unless a certification applies, the buyer withholds $4m at closing. The pension later files to settle its actual tax on the ECI portion of its gain.
Where you'll see it: Forms 8804, 8805, 8813; W-8IMY packages; secondary-sale transfer documents and 1446(f) certifications.
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This glossary is general information, simplified for learning. It is not tax, legal or investment advice, and real structures turn on facts and documents not shown here.