The aggregation rules are a set of rules that combine the ownership of the shareholders who own less than five percent of the stock of a loss corporation. Specifically, under the aggregation rules, the ownership of public shareholders, public owners, and 5-percent owners is combined to form one or more public groups of (1) the loss corporation or (2) of an entity that owns stock of the loss corporation (such as first tier entity or higher tier entity).
See Treas. Reg. § 1.382-2T(j)(1).