Based on what I understood from reading of the “jBPM5 Developer Guide” book Service Task is designed for integrating when you run within SOA implementation. The main parameters are Interface and Operation. Operation is a method on the Interface to be invoked. The Abstract Task is an extension point where you define what object you wish to call and bind it via Task Name. The Task Name is used when you register a handler for your task.
Hope this helps. Someone may want to correct me if I’m wrong.
Thanks,
Vladimir