25 Pa. Code § 145.80
A unit that is not a NOx budget unit under § 145.4(a) (relating to applicability), is not a unit exempt under § 145.4(b), vents all of its emissions to a stack, and is operating, may qualify, under this section and §§ 145.81-145.88 (relating to opt-in process), to become a NOx budget opt-in source. A unit that is a NOx budget unit under § 145.4(a), is covered by a retired unit exemption under §§ 145.4(b) or 145.5 (relating to retired unit exemption) that is in effect, or is not operating is not eligible to become a NOx budget opt-in unit.
25 Pa. Code § 145.80
This section cited in 25 Pa. Code § 129.201 (relating to boilers); 25 Pa. Code § 129.202 (relating to stationary combustion turbines); 25 Pa. Code § 145.2 (relating to definitions); 25 Pa. Code § 145.5 (relating to retired unit exemption); 25 Pa. Code § 145.50 (relating to NOx Allowance Tracking System accounts); 25 Pa. Code § 145.54 (relating to compliance); 25 Pa. Code § 145.55 (relating to banking); 25 Pa. Code § 145.70 (relating to general monitoring requirements); 25 Pa. Code § 145.83 (relating to applying for a NOx budget opt-in approval); and 25 Pa. Code § 145.84 (relating to opt-in process).