Maine's UIM is a difference-in-limits rule, and its stacking rule is left to your policy
Maine's rule is in 24-A MRSA 2902: subsection (1) defines uninsured and underinsured vehicles, subsection (2) sets the required limits and the rejection form, and subsection (6) sets how underinsured coverage is calculated. The minimum limits are in 29-A MRSA 1605, and the general deadline in 14 MRSA 752.
Read how stacked and single-limit coverage differ in Maine, check what section 2902 says and leaves open, or see how Maine's uninsured vehicle coverage works.