I use a separate notation for Types than for classes. Types are shown with a "Set-like" notation of a curly edged box. This distinguishes them from classes and makes them more distinct then just a rectangle again. Otherwise they have the same meaning as the rectangles in [Kilov+R 94].

Method (and Attribute) compartments can be specified for a Type, but instance variables can not be.

A Class (or Object) can be shown to implement a Type by an arrow pointing from the Class to the Type.

When the class is not important enough to be very large, its symbol can be shrunk and the name placed outside. This is frequently useful for giving example implementations of a Type in a complicated diagram.