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. See also section 401(a)(17) and 1.401(a)(17)-1(c)(1). For this purpose, in case of an HCE whose ACR is determined under 1.401(m)-2(a)(3)(ii), period of participation includes periods under another plan for which matching contributions or employee contributions are aggregated under 1.401(m)-2(a)(3)(ii).