29 C.F.R. § 4044.17

(a) General rule. A plan may establish one or more subclasses within any priority category, other than priority categories 1 and 2, which subclasses will govern the allocation of assets within that priority category. The subclasses may be based only on a participant's longer service, older age, or disability, or any combination thereof.

(b) Limitation. Except as provided in paragraph (c) of this section, whenever the allocation within a priority category on the basis of the subclasses established by the plan increases or decreases the cumulative amount of assets that otherwise would be allocated to guaranteed benefits, the assets so shifted shall be reallocated to other participants' benefits within the priority category in accordance with the subclasses.

(c) Exception for subclasses in effect on September 2, 1974. A plan administrator may allocate assets to subclasses within any priority category, other than priority categories 1 and 2, without regard to the limitation in paragraph (b) of this section if, on September 2, 1974, the plan provided for allocation of plan assets upon termination of the plan based on a participant's longer service, older age, or disability, or any combination thereof, and—

(d) Discrimination under Code. Notwithstanding the provisions of paragraphs (a) through (c) of this section, allocation of assets to subclasses established under this section is permitted only to the extent that the allocation does not result in discrimination prohibited under the Code and regulations thereunder.