p. 17 “Yee and colleagues (2003) define facets as “orthogonal sets of categories.” They note that facets may be either flat (containing a single level of values) or hierarchical (containing multiple levels of values in an ancestor-descendant structure). Furthermore, facets may be single-valued (allowing just one value to be assigned to an item) or multi-valued (allowing more than one value to be assigned to an item)…This specification of facets makes useful basic distinctions. However, a more general way of thinking about using facet classification is to model an information space that contains resources, which are described by facets (which are composed of schemes and structures).”