Understanding eligibility and requirements for ETIAS
ETIAS Visa Waiver is mandatory for trips to such countries whose purpose is either business, tourism, transit and medical treatment provided of course that the traveller holds a passport of a Visa Waiver eligible country. But even with such binding preliminary conditions ETIAS is not valid for all sorts of travels. Another paramount conditions is indeed that the total stay within the ETIAS Area as a whole must never exceed 90 days inside a 6 months period.
Hence travellers wishing to stay for longer periods of time and/or that plan to study or work in an ETIAS country will still have to apply for a specific Consular Visa. That naturally also includes, regardless of purpose and duration of the journey, all those travellers that do not have Visa Waiver rights to the Schengen Area. On the other hand, holding a valid ETIAS Visa Waiver is not required travel condition to citizens of any country if they already possess a valid Schengen Visa or a Permit of Stay issued by one of the 29 States that currently belong in the Schengen Area or its 4 de facto Members (therefore currently excluding the 1 EU Nation, Cyprus, that is bound to adhere to the border free travel area in the near future).
While not being suitable for long term study programs in the EU / Schengen Area, ETIAS Visa Waiver can be used for short term study programs (such as company arranged training classes, language classes and cooking classes) if the traveller is a National one of the 60 Visa Waiver eligible countries and if the stay in the ETIAS countries area will not exceed the aforementioned threshold of 90 days within 180 days.