Re: What are these EASA Acronymns?
I guess the main difference is that a STC is not for a single part, a STC can e.g. cover the complete interior of a business jet (if everything is modified and personalized).
But you could additionally get an ETSO for every single part of that same modification, e.g. every screw, hinge, display, etc.
I worked 7 years as a Powerplant Engineer for an international commercial airline based in middle europe and I never came across the Term ETSO. But engines are not often modified by an operator himself (as against the passenger cabin for example) so all parts you are replacing/repairing/inspecting are from the OEM of the engine and described in the manuals.