The UML is a language for creating software design diagrams. It is a design tool. The aspect of a
multi-dimensional array would be categorized as an implementation detail. Details such as these are
not that important in the design phase and there is no specific notational symbol for it. However, there
are mechanisms in the UML for you to specify such information. You could create a Note and place the Note
on the diagram specifying that variable x is a multi-dimensional array. Or you could include a stereotype
on the Class and specify the information there.