(a) Imposition of tax. (1) This section applies for purposes of determining the tax of a foreign corporation which at no time during the taxable year is engaged in trade or business in the United States. However, see also § 1.882–2 where such corporation has an election in effect for the taxable year in respect to real property income or receives interest on obligations of the United States. Except as otherwise provided in § 1.871–12, a foreign corporation to which this section applies is not subject to the tax imposed by section 11 or section 1201(a) but, pursuant to the provisions of section 881(a), is liable to a flat tax of 30 percent upon the aggregate of the amounts determined under paragraphs (b) and (c) of this section which are received during the taxable year from sources within the United States. Except as specifically provided in such paragraphs, such amounts do not include gains from the sale or exchange of property. To determine the source of such amounts, see sections 861 through 863, and the regulations thereunder.