5 C.F.R. § 2635.303
For purposes of this subpart, the following definitions apply:
(b) Indirectly, for purposes of § 2635.302(b), has the meaning set forth in § 2635.203(f). For purposes of § 2635.302(a), it includes a gift:
(f) Voluntary contribution means a contribution given freely, without pressure or coercion. A contribution is not voluntary unless it is made in an amount determined by the contributing employee, except that when an amount for a gift is included in the cost for a luncheon, reception, or similar event, an employee who freely chooses to pay a proportionate share of the total cost in order to attend will be deemed to have made a voluntary contribution. Except in the case of contributions for a gift included in the cost of a luncheon, reception, or similar event, a statement that an employee may choose to contribute less or not at all must accompany any recommendation of an amount to be contributed for a gift to an official superior.
Example 1 to paragraph (f): A supervisory employee of the Agency for International Development has just been reassigned from Washington, DC, to a foreign duty location. As a farewell party, 12 subordinates have decided to take the supervisory employee out to lunch at a restaurant. It is understood that the employees will pay for their own meals and that the cost of the supervisor's lunch will be divided equally among the 12. Even though the amount they will contribute is not determined until the supervisor orders lunch, the contribution made by those who choose to participate in the farewell lunch is voluntary.