
You need a national D visa whenever your intended stay in Switzerland exceeds 90 days, for example for work, study or family reunification. Short visits up to 90 days are covered by a Schengen (C) visa or visa-free entry.
The Schengen short-stay visa only allows you to be in Switzerland and the wider Schengen area for up to 90 days in any 180-day period, which is meant for tourism, short business trips and family visits. As soon as your purpose is to live in Switzerland for longer than that, you fall outside the short-stay rules.
For long stays a category D (national) visa is required, and it is tied to a specific purpose such as employment, studies or joining a family member. The cantonal migration office assesses the underlying purpose first, and the D visa is then issued by a Swiss representation abroad so you can travel to Switzerland and pick up your residence permit.
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