A SDI is a public access system of (political) measures, institutional facilities, technologies, data and people to provide a common exchange and efficient use of geographical information.
SDIs exist on different levels. Communes, cities and cantons could install SDIs, the Federal Spatial Data Infrastructure for instance concerns the geodata of the Confederation, the National Spatial Data Infrastructure includes public organisations as well as private entreprises. On the European level we find Eurogeographics and INSPIRE.
Please find more information on this topic on the geoportal of the Swiss Confederation: