A: The link between 14th-Amendment citizenship and the IRS is found in the Code of Federal Regulations, specifically 26 CFR 1.1-1(c).

Let's review: What is the CFR? Congress writes the statutes, then delegates some of the details to the Secretary of the Treasury, who in turn delegates them to some bureaucrat, who writes Federal Regulations which are published in the Federal Register, which constitutes notice to every person in the Continental United States according to the Federal Register Act and the US Supreme Court case of Merrill vs. Federal Crop Insurance. (The dissenting opinion said that if the farmer had read everything in the Federal Register, he wouldn't need to buy crop insurance since he wouldn't have any time for planting crops!)

Now back to our subject: 26 CFR 1.1-1(c) defines "citizen" using the same words as the 14th Amendment, with an interesting twist:

The phrase "United States" can be either singular or plural. When the Union was formed, it was understood that the "citizens" were citizens of the States. "Technically and abstractly there was no such thing as a citizen of the United States." The 14th Amendment says that all persons born or naturalized in the United States and subject to "the jurisdiction thereof" are citizens of the United States [i.e. federal citizens] and of the States in which they reside. Notice what it does NOT say. It doesn't say "subject to THEIR jurisdiction" (this would refer to the jurisdiction of the State governments) and it doesn't say "subject to ITS jurisdiction" (meaning Federal jurisdiction). It is deliberately ambiguous. Now, 26 CFR 1.1-1(c) says "subject to ITS jurisdiction" showing that it clearly refers to federal citizens.

The purpose of expatriation/repatriation is to sever your link to 26 CFR 1.1-1(c) [or to put it another way, to remove all doubt whether there is a link or not] whilst affirming that you are a State citizen, in other words, a pre-Civil War-type citizen. Also known as an American National.