U heeft niet aangetoond dat u (fiscaal) ingezetene bent van een ander land dan Nederland. Daarom kunnen we geen verdrag ter voorkoming van dubble belasting toepassen en blijft u onderworpen aan de Nederlands fiscale wetgeving
我也不是很懂,用google翻译大概意思是:
You have not demonstrated your (tax) resident of a country other than the Netherlands. Therefore we can not Convention against dubble taxes apply and you remain subject to Dutch tax law
if you finish your PhD in another country you can provide your old salary certification ( like how much you get per month or per year) to the tax office. They need to know you get salary before you come to the Netherlands.
I think you can contact the HRM office in your University. They can help you. If you did your PhD in another country you should get 30% ruling. 30% is different from what we called "tax refund". We can apply tax refund by ourselves each year but 30% must be applied by your employer, not by yourself.