18-125-801 Me. Code R. § 03
If the business activity of a taxpayer occurs both within and without Maine, and if by reason of such activity the taxpayer is taxable in another state, the portion of the net income (or net loss) derived from sources within Maine is determined by apportionment in accordance with 36 M.R.S. §§5142(6) and 5210-11 and the provisions of this rule. A corporation or affiliated group of corporations may be engaged in more than one unitary business. In that event, the corporation or affiliated group of corporations must, for each line of business, separately apportion its income using the appropriate Maine apportionment factor. Maine utilizes a "water's edge" combined reporting methodology for determining the apportionable income base. The income subject to apportionment is income required to be reported on the taxpayer's federal income tax return as modified by Maine law. The apportionment factor must only include those amounts attributable to the apportionable income base for that taxable year. Variations may be allowed when petitioned for by the taxpayer or may be required by the assessor. 36 M.R.S. §5211(17).
18-125 C.M.R. ch. 801, § 03