(a) For purposes of the CIWMB promulgated sections of this article, “closed” refers to the status of a disposal site that either 1) has received a closure certification pursuant to § 21880, or 2) has, on or before November 18, 1990, completed all closure activities required pursuant to regulations in effect at the time of the last receipt of waste.

(b) For purposes of the CIWMB promulgated sections of this article, “closing” means the period that commences when implementation of an approved final closure or partial final closure plan begins, and that ends when implementation of an approved final closure or partial final closure plan is complete.