14 CFR § 241.2-3 - Sec. 2-3 Distribution of revenues and expenses within entities.
(a) Revenues and expenses attributable to a single natural objective account or functional classification shall be assigned accordingly.
(b) Revenue and expense items which are common to two or more natural objective accounts shall be recorded in the objective accounts to which they predominantly relate.
(c) Expense items contributing to more than one function shall be charged to the general overhead functions to which applicable except that where only incidental contribution is made to more than a single function an item may be included in the function to which primarily related, provided such function is not distorted by including an aggregation of amounts applicable to other functions. When assignment of expense items on the basis of the primary activity to which related does not in the aggregate result in a fair presentation of the expenses applicable to each function, apportionment shall be made between functions based upon a study of the contribution to each function during a representative period.