The ExpressionGuardVariableTypeprovides, in addition to the human-readable semantic from its GuardVariableType, a machine interpretable representation on the semantic.

The ExpressionGuardVariableTypeis formally defined in Table 21.

Table 21– ExpressionGuardVariableType definition

Attribute	Value
BrowseName	ExpressionGuardVariableType
DataType	LocalizedText
ValueRank	−1 (−1 = Scalar)
IsAbstract	False
References	NodeClass	BrowseName	DataType	TypeDefinition		ModellingRule
Subtype of the GuardVariableType defined in Clause 4.6.4
HasProperty	Variable	Expression	ContentFilter		PropertyType		Mandatory
Conformance Units
Base Info Choice States

The mandatory Property Expressionprovides a ContentFilter, that shall be evaluated on the Object the StateMachine belongs to. This is a machine-readable semantic of the Guard. If the ContentFilterevaluates to "True", the Guardis "True", otherwise the Guardis "False".

The ContentFilterfor ExpressionGuardVariableTypeis restricted to basic operators (see OPC 10000-4for details).

The ContentFiltercan reference any Variablesdefined on the StateMachineTypeand are validated on the instance of the StateMachine. If the definition of the Variableis not owned directly by the StateMachinebut for example some other type, the StateMachineinstance shall reference that Variable.