15 U.S.C. § 381: Imposition of net income tax

the solicitation of orders by such person, or his representative, in such State for sales of tangible personal property, which orders are sent outside the State for approval or rejection, and, if approved, are filled by shipment or delivery from a point outside the State; and the solicitation of orders by such person, or his representative, in such State in the name of or for the benefit of a prospective customer of such person, if orders by such customer to such person to enable such customer to fill orders resulting from such solicitation are orders described in paragraph (1). any corporation which is incorporated under the laws of such State; or any individual who, under the laws of such State, is domiciled in, or a resident of, such State. For purposes of subsection (a), a person shall not be considered to have engaged in business activities within a State during any taxable year merely by reason of sales in such State, or the solicitation of orders for sales in such State, of tangible personal property on behalf of such person by one or more independent contractors, or by reason of the maintenance, of an office in such State by one or more independent contractors whose activities on behalf of such person in such State consist solely of making sales, or soliciting orders for sales, or tangible personal property. the term “independent contractor” means a commission agent, broker, or other independent contractor who is engaged in selling, or soliciting orders for the sale of, tangible personal property for more than one principal and who holds himself out as such in the regular course of his business activities; and the term “representative” does not include an independent contractor.