What are abstract symbols?
Some UML types have a large number of specializations. For example, the UML type Action has about 40 specializations, such as CallOperationAction or OpaqueAction. Instead of providing 40 different Action symbols in the Symbols bar of the activity diagram, the bar initially contains only one symbol for the abstract type Action, so that the complexity of the Symbols bar is reduced.
When placing this abstract Action symbol in the diagram, the user is asked to specify the relevant action to be created. In addition to or as an alternative to the abstract Action symbol you can also add specific Action symbols to the Symbols bar, such as CallOperationAction or OpaqueAction.