Two types of Variablesare defined, Propertiesand DataVariables. Although they differ in the way they are used as described in 4.4and have different constraints described in the remainder of 5.6they use the same NodeClassdescribed in 5.6.2. The constraints of Propertiesbased on this NodeClassare defined in 5.6.3, the constraints of DataVariables in 5.6.4.