The part that lets countries cooperate on cutting emissions, including by trading them. One country pays for a reduction in another and counts it toward its own target. It took nine years to agree the rules.
Article 6 has three working parts. Article 6.2 covers direct deals between countries and sets the accounting rules so one tonne is not counted twice. Article 6.4 sets up a centralised UN crediting mechanism, the closest thing the Paris Agreement has to a global carbon market. Article 6.8 covers cooperation that does not involve trading at all.
The standards were finally adopted at COP29 in Baku in November 2024, after the subject had collapsed at three previous conferences. That is the reason it dominated so much coverage: not because the idea was new, but because the rules for it had been unresolved since 2015.
The argument that remains is about integrity. A credit only works if the reduction it represents is real, additional and permanent, and the history of voluntary carbon markets gives plenty of reason for scepticism.