For purposes of subdivision (ii) of this subparagraph, the number of employers maintaining the plan is determined by treating any employers described in section 414(b) (relating to a controlled group of corporations) or any employers described in section 414(c) (relating to trades or businesses under common control), whichever is applicable, as if such employers are a single employer. See § 1.411(a)-5(b)(3) for rules relating to the time when an employer maintains a plan. A master or prototype plan is not a section 413(c) plan unless such a plan is described in this subparagraph. Similarly, the mere fact that a plan, or plans, utilizes a common trust fund or otherwise pools plan assets for investment purposes does not, by itself, result in a particular plan being treated as a section 413(c) plan.
26 C.F.R. §1.413-2
Sec. 411 (88 Stat. 901; 26 U.S.C. 411 )