... but to understand it, we have to have the context of history.
Before the 20th century, most businesses had a legal structure of either sole proprietorship, partnership, or trust. Corporations were not used like they are today because many states did not recognize them and of those that did, many had laws restricting what they could do.
This is why the term "Massachusetts Business Trust" comes up sometimes. Massachusetts had a law forbidding corporations from owning real estate, so people used trusts instead.
The terms "trust busting" and "anti-trust" were used by the statists when they wanted to break up Standard Oil Company, which was a series of business trusts, not corporations.
Sometimes in the liberty movement we run across terms like "pure trust." This term was uttered by the United States Supreme Court in the case of Morrissey vs. Commissioner (1935) when they determined how business trusts or, they called it a "pure trust," would be taxed.
This is the court case where the term "an association taxable as a corporation" comes from. This is why the IRS has a "check the box" form (Form 8832). They MUST abide by the Morrissey case, but they don't want to, which is why there is a lot of propaganda out there.
The point is that before the 20th century, corporations were not used all that much. Some, but not like today. But as they were becoming more and more common, more and more states were passing laws allowing their creation. Since they are creatures of the state, they must follow the state's laws.
Congress passed the Corporation Excise Tax of 1909, which taxed the "privilege" of "doing business in corporate form," and it was a tax on net profits. The law said:
"That every corporation, joint stock company or association, organized for profit and having a capital stock represented by shares ... now or hereafter organized under the laws of the United States or of any State ... shall be subject to pay annually a special excise tax with respect to carrying on or doing business by such corporation ... equivalent to one per centum on the entire net income over and above five thousand dollars received by it from all sources during such year...."
This was upheld as constitutional by the SCOTUS in Flint vs. Stone Tracy (1911).
Right after that, Congress passed the 16th Amendment and the Income Tax Act of 1913. The 16th was never properly ratified and should not be a part of the Constitution, but the law business is corrupt.
In 1926, the SCOTUS ruled in the Bowers v. Kerbaugh-Empire case that:
"The Sixteenth Amendment declares that Congress shall have power to levy and collect taxes on income, 'from whatever source derived' without apportionment among the several states, and without regard to any census or enumeration. It was not the purpose or effect of that amendment to bring any new subject within the taxing power. Congress already had power to tax all incomes. But taxes on incomes from some sources had been held to be 'direct taxes' within the meaning of the constitutional requirement as to apportionment. Art. 1, 2, cl. 3, 9, cl. 4; Pollock v. Farmers' Loan & Trust Co., 158 U.S. 601 , 15 S. Ct. 912. The Amendment relieved from that requirement and obliterated the distinction in that respect between taxes on income that are direct taxes and those that are not, and so put on the same basis all incomes 'from whatever source derived.' Brushaber v. Union Pac. R. R., 240 U.S. 1, 17 , 36 S. Ct. 236, 241 (60 L. Ed. 493, L. R. A. 1917D, 414, Ann. Cas. 1917B, 713). 'Income' has been taken to mean the same thing as used in the Corporation Excise Tax Act of 1909 (36 Stat. 112), in the Sixteenth Amendment, and in the various revenue acts subsequently passed. Southern Pacific Co. v. Lowe, 247 U.S. 330, 335 , 38 S. Ct. 540; Merchants' L. & T. Co. v. Smietanka, 255 U.S. 509, 219 , 41 S. Ct. 386, 15 A. L. R. 1305. After full consideration, this court declared that income may be defined as gain derived from capital, from labor, or from both combined, including profit gained through sale or conversion of capital. Stratton's Independence v. Howbert, 231 U.S. 399, 415 , 34 S. Ct. 136; Doyle v. Mitchell Brothers Co., 247 U.S. 179, 185 , 38 S. Ct. 467; Eisner v. Macomber, 252 U.S. 189, 207 , 40 S. Ct. 189, 9 A. L. R. 1570. And that definition has been adhered to and applied repeatedly. See, e. g., Merchants' L. & T. Co. v. Smietanka, supra, 518 (41 S. Ct. 386); Goodrich v. Edwards, 255 U.S. 527, 535 , 41 S. Ct. 390; United States v. Phellis, 257 U.S. 156, 169 , 42 S. Ct. 63; Miles v. Safe Deposit Co., 259 U.S. 247, 252 , 253 S., 42 S. Ct. 483; United States v. Supplee-Biddle Co., 265 U.S. 189, 194 , 44 S. Ct. 546; Irwin v. Gavit, 268 U.S. 161, 167, 45 S. Ct. 475; Edwards v. Cuba Railroad, 268 U.S. 628, 633 , 45 S. Ct. 614. In determining what constitutes income substance rather than form is to be given controlling weight. Eisner v. Macomber, supra, 206 (40 S. Ct. 189)."
Is that the answer you were looking for? ;-)