Legend Classes (data types) are shown as boxes. Abstract classes are denoted by dotted outline; “identifiable” classes, which may be referenced with an identifier, are denoted by bold borders; non-identifiable classes are denoted by thin solid borders. Inheritance and composition are shown with dotted gray and solid black lines connecting classes, respectively. All classes have a string type. Abstract classes enable specializations of concepts in this and future versions of VRS. Identifiable classes have an optional _id attribute. Non-identifiable classes exist only to structure data always occur “inline” within objects. Inherited attributes are not shown in this diagram. An asterisk (*) denotes a nullable attribute. A dagger (†) denotes attributes that may be specified with inline objects or references to the same type.