What is an ETSO approval and where can I find the corresponding regulations?
The ETSO (European Technical Standard Order) authorization represents one way (and not the only way) to have parts and appliances approved. This is an optional step which ensures that a part or appliance complies with a minimum performance standard. In all cases, the installer must apply for an installation approval on-board the aircraft. He/she can use the ETSO authorization and he/she complements it to demonstrate that the installation complies with the applicable certification basis for the changed aircraft.
European Technical Standard Order authorizations are covered by Subpart O of Part-21 which is the Annex to Commission Regulation (EU) No 748/2012. You can find a list of the ETSO standards in the published list ETSO Index. The certification specifications for ETSO are published here.