D.C. Mun. Regs. tit. 9, r. 9-125
All business income shall be apportioned to the District by multiplying the income by a fraction, the numerator of which is the property factor, plus the payroll factor, plus the sales factor, and the denominator of which is three (3), reduced by the number of factors, if any, having no denominator.
Rental income from real and tangible property shall be business income if the property with respect to which the rental income was received is used in the taxpayer's trade or business or is incidental thereto.
Gain or loss from the sale, exchange or other disposition of real, tangible or intangible personal property, shall constitute business income if the property was owned or leased by the taxpayer in the trade or business.
Interest income shall be business income where the intangible with respect to which the interest was received arises out of or was created in the regular course of the taxpayer's trade or business, or where the purpose for acquiring or holding the intangible is related to or incidental to the trade or business operations.
Dividends shall be business income where the stock with respect to which the dividends are received arises out of or was acquired in the regular course of the taxpayer's trade or business operations or where the purpose for acquiring and holding the stock is related to or incidental to the trade or business.
Patent and copyright royalties shall be business income where the patent or copyright with respect to which the royalties were received arises out of or was created in the regular course of the taxpayer's trade or business operations or where the purpose for acquiring and holding the patent or copyright is related to or incidental to the trade or business operation.
D.C. Mun. Regs. tit. 9, r. 9-125