There are several types of international voluntary emissions reductions, this article will focus on two significant types: the Gold Standard (GS) and the Verified Carbon Standard (VCS).

1. Gold Standard (GS): Developed by the Gold Standard Foundation, GS certifies and issues two types of voluntary carbon credits. One carbon credit represents the reduction or elimination of one ton of carbon dioxide equivalent. These include: (1) GS-CER: Certified emission reductions based on action projects developed under the Clean Development Mechanism (CDM) and certified by the gold standard. (2) GS-VER: Voluntary emission reductions based on projects developed according to the gold standard's own criteria. Carbon credits issued and verified by Gold Standard can be traded on the Gold Standard trading platform or through OTC transactions.
2. Verified Carbon Standard (VCS): Initiated by climate groups, the International Emissions Trading Association, the World Economic Forum and the World Trade Council for Sustainable Development, the Verified Carbon Standard is one of the most widely used international carbon credit standards. Voluntary emission reductions or carbon credits verified and issued by VCS are known as Verified Carbon Units (VCU), with each VCU representing a reduction or offset of one tonne of carbon dioxide equivalent. Unlike GS, VCUs can only be traded within the Verra VCS system and cannot be traded off-exchange between buyers and sellers.