In most cases Variables have the TypeDefinition DataItemType or one of its subtypes. The optional Property Definition can be added to a Variable that uses such a TypeDefinition. This allows manufacturers to store a specific definition for each Variable.
Variables that use the DataType Boolean are modeled with the TypeDefinition TwoStateDiscreteType. Such Variables have the TrueState and FalseState Properties, which provide human readable and mandatory True and False states in their Value Attribute.