Choose as many criteria as you like to build your search. If you select multiple criteria of the same type, the search function will return results that match any of the criteria. If you select multiple criteria of different types, the search function will return results that match any of the criteria within a type, but the results must match ALL of the criteria types. The more criteria of the same type you add, the more general the results will be. The more types of criteria you add, the more specific the results will be.
For example, if you choose the classifications "Painting" and "Tapestries", and also the museums "France, Paris: Musee du Louvre" and "United States, New York, NY: Metropolitan Museum of Art", the results will be all works that are paintings OR tapestries AND are in either the Louvre, OR the Met.