16 C.F.R. § 240.5
Competing customers are all businesses that compete in the resale of the seller's products of like grade and quality at the same functional level of distribution regardless of whether they purchase directly from the seller or through some intermediary.
Example 1: Manufacturer A, located in Wisconsin and distributing shoes nationally, sells shoes to three competing retailers that sell only in the Roanoke, Virginia area. Manufacturer A has no other customers selling in Roanoke or its vicinity. If Manufacturer A offers its promotion to one Roanoke customer, it should include all three, but it can limit the promotion to them. The trade area should be drawn to include retailers who compete.
Example 2: A national seller has direct-buying retailing customers reselling exclusively within the Baltimore area, and other customers within the area purchasing through wholesalers. The seller may lawfully engage in a promotional campaign confined to the Baltimore area, provided that it affords all of its retailing customers within the area the opportunity to participate, including those that purchase through wholesalers.
Example 3: B manufactures and sells a brand of laundry detergent for home use. In one metropolitan area, B's detergent is sold by a grocery store and a discount department store. If these stores compete with each other, any allowance, service or facility that B makes available to the grocery store should also be made available on proportionally equal terms to the discount department store.
16 C.F.R. §240.5