TaxGuyBill
Level 15
12-06-2019
08:10 PM
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In answer to the question, I think the best "official" resource (which will stand up in court) is the preamble to the Proposed Regulations, and starting on page 57 it goes through each of those categories.
https://www.irs.gov/pub/irs-drop/reg-107892-18.pdf#page=57