The standard allowance, an “assigned amount unit” (AAU), is equivalent to one metric ton of carbon dioxide emissions. […] In general, these credits can be purchased, sold, and otherwise transferred by private actors through international markets. Unlike AAUs, they are not allocated by a state but rather are generated through voluntary activities undertaken most commonly by private actors. 2014, John G Sprankling, “Rights in Airspace and Outer Space”, in The International Law of Property, Oxford University Press, →ISBN, page 174: