An Assigned Amount Unit (AAU) is a tradable 'Kyoto unit' or 'carbon credit' representing an allowance to emit greenhouse gases comprising one metric tonne of carbon dioxide equivalents calculated using their Global Warming Potential. Assigned Amount Units are issued up to the level of initial "assigned amount" of an Annex 1 Party to the Kyoto Protocol.
The "assigned amounts" are the Kyoto Protocol Annex B emission targets (or "quantified emission limitation and reduction objectives") expressed as levels of allowed emissions over the 2008-2012 commitment period.
Article 17 of the Kyoto Protocol allows emissions trading between Annex B Parties (countries). Parties that have "assigned amount units" to spare because of reductions in emissions below their Kyoto commitment set out in Article 3 and Annex B may sell those units to countries that have emissions exceeding their targets. Article 17 also requires that any such emissions trading shall be supplemental to domestic action for the purpose of meeting quantified emission limitation and reduction commitments (QELRCs).