An OpaqueExpression in UML 2 is used to specify behavior in a textual form when it is not necessary to specify which language is used, or when it is not possible to use graphical notation. According to the UML 2 specification, an OpaqueExpression is not limited to any particular language; it can represent expressions written in any language.
The key characteristics of an OpaqueExpression are as follows:
* The 'body' of an OpaqueExpression is a sequence of strings, where each string could be written in a different language. This means it has the capacity to include multiple languages simultaneously.
* The 'language' attribute is optional and is used to indicate the languages of the respective 'body' parts.
The allowance for any language is clearly stated in the UML 2 Infrastructure Specification. OpaqueExpression is designed to be a flexible way to capture expressions that are not readily expressible in UML or when using a language outside the scope of UML (e.g., mathematical expressions, programming languages, or other domain-specific languages).