AggregationKind is an enumeration type that specifies the literals for defining the kind of aggregation of a property. VisibilityKind is an enumeration type that defines literals to determine the visibility of elements in a model. A comment is a textual annotation that can be attached to a set of elements. A literal integer is a specification of an integer value. A literal unlimited natural is a specification of an unlimited natural number. Note that this is either the string 'unbounded' or a non-negative integer. A package is used to group elements, and provides a namespace for the grouped elements. The Model construct is defined as a Package. It contains a (hierarchical) set of elements that together describe the physical system being modeled. A Model may also contain a set of elements that represents the environment of the system, typically Actors, together with their interrelationships, such as Associations and Dependencies. zOOml Modeler requires a single Package inside the (XMI child) Model element to represent (and bound) the system or subsystem. A property is a structural feature of a classifier that characterizes instances of the classifier. A property related by ownedAttribute to a classifier (other than an association) represents an attribute and might also represent an association end. It relates an instance of the class to a value or set of values of the type of the attribute. A property related by memberEnd or its specializations to an association represents an end of the association. The type of the property is the type of the end of the association. An operation is a behavioral feature of a classifier that specifies the name, type, parameters, and constraints for invoking an associated behavior. A class describes a set of objects that share the same specifications of features, constraints, and semantics. An association describes a set of tuples whose values refer to typed instances. An instance of an association is called a link. A generalization is a taxonomic relationship between a more general classifier and a more specific classifier. Each instance of the specific classifier is also an indirect instance of the general classifier. Thus, the specific classifier inherits the features of the more general classifier. A dependency is a relationship that signifies that a single or a set of model elements requires other model elements for their specification or implementation. This means that the complete semantics of the depending elements is either semantically or structurally dependent on the definition of the supplier element(s).