An abstract data type (ADT) is a user-defined data type which can encapsulate a data set and functions that can be performed on the data. Abstract data types are not supported by all DBMSs.
For example you could create an abstract data type for the Gregorian calendar to read and write roman numerals and convert dates between the Julian and Gregorian calendars.
If your model contains abstract data types of type JAVA, you can link them to Java classes in an OOM to model and review the Java class properties (see Linking an Abstract Data Type to a Java Class).