Some DBMS like Oracle or DB2 Common Server support tables based on abstract data types (ADT). A table based on an ADT uses the properties of the ADT and the ADT attributes become table columns.
To link a table to an ADT you have to use the Based On list to select an abstract data type. Not all ADT can be used, only ADT of type Object in Oracle, or Structured in DB2 Common Server appear in the Based On list.
For more information on abstract data types, see Abstract Data Types (PDM).