D.C. Mun. Regs. tit. 9, r. 9-116
The taxable income of corporations and financial institutions under Title 7 is defined to mean "the amount of net income derived from sources within the District within the meaning of Title 10 of the Act;" therefore, to determine what is "taxable income" of financial institutions and corporations, it is necessary to apply the provisions of Title 10 of the Act and §§ 121 through 129 of this chapter.
The tax imposed upon corporations and financial institutions is levied for the privilege of carrying on or engaging in a trade or business within the District, and of receiving other income which is derived from sources within the District.
The term "corporation" includes any trust, association, join stock company, or partnership classed (or which should be classed) as a corporation for purposes of federal income taxation.
D.C. Mun. Regs. tit. 9, r. 9-116