That you for your feedbacks.
I will send them a letter.
I would like to add official sources mentioning it.
I have found S-02.142 document on the subject but only for qualified intermediary (IB).
It is available in German for the English website but you have also French.
However there is no mention of L1 nor L2.
Have you got a better source for private investor Swiss resident ?
I am reading the main thread but the main source is boggle heads for Canadian investor.
@s-g mentioned another article that should be enough for the us :
https://www.estv.admin.ch/estv/fr/accueil/droit-fiscal-international/international-par-pays/sif/impots-source-selon-cdi/auslaendische-quellensteuern-pro-land.html#-591145893