Fungible, or Fungibility
Being of such a nature that one part or quantity may be replaced by another equal part or quantity in the satisfaction of an obligation. The fungibility of a ton of carbon dioxide equivalent (CO2e) for an example allows for the use of various types of gas i.e Methane. Fungibility can also allow for the possibility to exchange, or use interchangeably, different types of reduction credits achieved under different mechanism (e.g. CERs for AAUs).
"Glossary", Eco securities Limited. "Challenges for a business case for high-biodiversity REDD Projects and Schemes."- A Report for the Secretariat of the CBD, February 2009, Version1.2 & Global Canopy Programme, "Glossary of Terms" - "The Little REDD Book. A Guide to governmental and nongovernmental proposals for reducing emissions from deforestation and degradation." (Global Canopy Foundation, November 2008.)
The property of a REDD+ credit being freely exchangeable/substitutable with other types of carbon credits, for example carbon credits derived from CDM and JI projects.