The US Coast Guard Office of Commercial Vessel Compliance issued a circular to outline how the Coast Guard will enforce MARPOL Annex VI Regulation 13.5.1.2 due to the unavailability of engines needed to comply with this regulation.
MARPOL Annex VI Regulation 13.5.1.2 generally prohibits the operation of any engine in the North American and US Caribbean Sea NOx Emission Control Areas (ECA) that does not comply with the MARPOL Annex VI Tier III requirements if the engine is installed on a ship...
https://safety4sea.com/uscg-engine-types-accepted-under-marpol-annex-vi/