26 C.F.R. § 36.3121(l)(9)-1
A domestic corporation which enters into an agreement as provided in § 36.3121(l)(1) -1 shall, for purposes of the regulations in this part and for purposes of section 6413(c)(2)(C), relating to special credits or refunds, be considered an employer in its capacity as a party to such agreement separate and apart from its identity as an employer incurring liability for the employee tax and employer tax on the wages of its own employees. Thus, if a citizen of the United States performs services in employment for the domestic corporation and at any time within the same calendar year performs services covered by the agreement as an employee of one or more foreign subsidiaries named therein, the limitation on wages provided in section 3121(a) (1) has application separately as to the wages for employment performed in the employ of the domestic corporation and as to the remuneration for services covered by the agreement performed in the employ of such foreign subsidiary or subsidiaries. All services covered by the agreement whether performed in the employ of one or more than one such foreign subsidiary are regarded for purposes of the wage limitation as having been performed in the employ of the domestic corporation in its separate capacity as a party to the agreement. Similarly, any remuneration for such services which, if the services were performed in the United States, would be excluded from wages unless a certain amount of such remuneration is paid by a single employer within a specified period (for example, remuneration for agricultural labor) is regarded, for purposes of determining whether the domestic corporation incurs liability under its agreement with respect to such remuneration, as having been paid by the domestic corporation in its separate capacity as a party to the agreement. All remuneration received by an employee for services covered by the agreement is deemed, for purposes of the special credit or refund provisions contained in section 6413(c), to have been received from the domestic corporation as an employer in its separate capacity as a party to the agreement.
26 C.F.R. §36.3121(l)(9)-1