Quota trading: refers to the trading of emission reduction units generated under total control, such as the "EU quota" (EUAs) trading of the EU emission trading system, which is mainly an excess emission reduction transaction between countries that have reduced their emissions under the Kyoto Protocol, usually a spot transaction. Project-based transaction: refers to the transaction of emission reduction units generated by emission reduction projects, such as "emission reduction warrants" under the clean development mechanism and "emission reduction units" under the joint implementation mechanism, which are mainly emission reduction transactions generated through emission reduction plans of cooperation between countries, and are usually bought and sold in advance in the form of futures.