A classifier is a category of Unified Modeling Language (UML) elements that have some common features, such as attributes or methods. A classifier is an abstract metaclass classification concept that serves as a mechanism to show interfaces, classes, datatypes and components. A classifier describes a set of instances that have common behavioral and structural features (operations and attributes, respectively). A classifier is a namespace whose members can specify a generalization hierarchy by referencing its general classifiers. All objects that can have instances are classifiers.