Discussion paper concerning the transposition of GTR 15 into a UN Regulation. The paper raises two possible routes: 1) a UN Regulation identical to EU legislation but that would not be not applied (in its entirety as a distinct rule) outside the EU; 2) a “hierarchical” UN Regulation that combines all elements of all emissions thresholds at their most stringent level with lower levels of approval corresponding to regional/national requirements. The EC proposes option 2 as this would allow for a global rule where Contracting Parties could certify systems to standards outside their markets (e.g., Japan could issue approvals to EU standards and vice versa) under the UN Regulation.