complex structures of topical categories task domains in the broadest sense should be understood as the groupings of all related schema or vocabularies as well as their constituent categories in a given task domain area subject area or semantic domain a task domain can correspond to one or more given vocabularies or xml document type definitions dtds or schema these terms are more or less used interchangeably these are referred to herein as generic task domains examples might be publications or real estate topical categories are specific types of information and their instance values such as the times as a publication and king as an author or apartments as a property type and los angeles as a location these may correspond to specific semantic categories within a vocabulary propertytypes in an rdf vocabulary or specific field type metadata tags in an xml schema further it is useful to distinguish types of categories or metadata or data fields from values or instances of categories or field types for example author or location is a type king or los angeles is a value in rdf a value would be a property u2014 the value of a propertytype for a resource thus within a generic task domain we can distinguish at least these four levels 1 the generic task domain or vocabulary or schema type 2 any of many specific instances of named vocabularies or schemas for example there may be many library catalog or purchase order vocabularies each with similar category metadata types and specific instances each with specifically defined category metadata types and allowed values 3 the specific category metadata types.