It is organized in the following form.
Subtitle B - Estate and Gift Taxes, chapters 11 -13
Subtitle C - Employment Taxes, chapters 21 - 25
Subtitle D - Miscellaneous Excise Taxes, chapters 31 -47
Subtitle E - Alcohol, Tobacco and Certain Other Excise Taxes, chapters 51 - 54
Subtitle F - Procedure and Administration, chpaters 61 - 80
Subtitle G - The Joint Comittee on Taxation
Subtitle H - Financing of Presidential election Campaign
Notice that Income Taxes are in Subtitle A, and it consists of only six chapters. Those six chapters comprise the entire extent of the income tax code (laws imposing the income tax).
Each subtitle within the title is a set of stand-alone laws that are separate and distinct in application from the other subtitles' laws. To make this clear I would ask: "Do you pay Subtitle E, alcohol and tobacco taxes ?". For most people, the answer is a resounding no, because subtitle E only affects those persons who manufacture or sell alcohol or tobacco. The "subjects" of Subtitle E are only those individuals.
The "subject" of Subtitle C is the group of individuals who were required, or have chosen voluntarily, to participate in the Social Security program.
The question then becomes: "Who are the subjects of the subtitle A income taxes ?"
To understand the answer to this question, one must carefully study the definition of a "Withholding Agent", where the subject of the tax (the person from whom the tax is to be withheld) is identified.
Withholding agents withhold income taxes under subtitle A.
Employers withhold employment taxes under subtitle C.
Who does a withholding agent have the authority to withhold from ? (seeSec. 7701. Withholding Agent & Definitions)
That group of people (foreigners) is the subject of the income tax imposed in subtitle A.