A unit that is in a State (as defined in § 97.2 ), is not a NOX Budget unit under § 97.4(a) , is not a unit exempt under § 97.4(b) , vents all of its emissions to a stack, and is operating, may qualify to be a NOX Budget opt-in unit under this subpart. A unit that is a NOX Budget unit under § 97.4(a) , is covered by an exemption under § 97.4(b) or § 97.5 that is in effect, or is not operating is not eligible to be a NOX Budget opt-in unit.
40 C.F.R. §97.80