Following the entry into force of the Agreement between the EU and Switzerland on the linking of their Emission Trading Systems on 1 January 2020, the emission allowances of the Emission Trading System of Switzerland became valid for compliance purposes in the EU Emissions Trading System and vice versa. The text of the Agreement is available here.

In order to enable the transfer of emission allowances between the two systems, an electronic link between the EU and Swiss emissions trading registries was launched on 21 September 2020. Since the registries are still not connected by a permanent direct link, through this provisional solution transfers to the other system (registry) will be performed on certain predefined dates. More information and predefined dates can be found on the website of European Commission.

In the Union Registry, a new type of international transaction is used to transfer allowances to Switzerland. All types of allowances can be transferred between accounts in the EU and Switzerland (European allowances EUA, European aircraft allowances EUAA, Swiss allowances CHU, Swiss aircraft allowances CHUA). Swiss allowances can also be transferred between EU accounts.

The transactions are initiated and approved under the same rules as are applied to internal transfers of allowances (ie between two EU accounts), including the delay (see the Transactions overview).

More information is available on the website of the European Commission: 
Frequently asked questions - part 1 and part 2
Decision No. 2/2019 of the Joint Committee - press release, text of the Decision