Compensation.Compensation means compensation as defined in section 414 and 1.414(s)-1. The period used to determine an employee's compensation for a plan year must be either the plan year or the calendar year ending within the plan year. Whichever period is selected must be applied uniformly to determine the compensation of every eligible employee under the plan for that plan year. A plan may, however, limit the period taken into account under either method to that portion of the plan year or calendar year in which the employee was an eligible employee, provided that this limit is applied uniformly to all eligible employees under the plan for the plan year. In the case of an HCE whose ADR is determined under 1.401(k)-2(a)(3)(ii), period of participation includes periods under another plan for which elective contributions are aggregated under 1.401(k)-2(a)(3)(ii). See also section 401(a)(17) and 1.401(a)(17)-1(c)(1).