Schengen Area
Some nationalities need visas to visit the Schengen Area.  They only need one Schengen visa for all countries.  But they must apply at the embassy of their main destination.  The main destination is where a traveller wants to spend the longest time.  The visa officer can issue a visa for only one trip or many trips.  The officer also decides how long the visa is valid for.  The traveller needs to exit before the Schengen visa expires.