In UML, ValueSpecifications are used to specify the value of an element. The UML 2 Specification does not define a metaclass named StringExpression. Instead, it provides a metaclass named OpaqueExpression, which can be used when an expression is written in a language that is not directly interpretable by the model. Since there is no metaclass called StringExpression in the UML 2 Specification, there is no explicitly intended use for it within the UML metamodel. The absence of this metaclass suggests that any use of a concept called
"StringExpression" would not conform to standard UML 2 practices and would likely be part of an extension or profile, not the core UML metamodel.