I've done a lot of work on a new New Term IBD-like diagram in Rhapsody for modeling Function Blocks as parts in a process flow. I've increasingly had to look to BPMN notation to provide a paradigm for how to manage aspects such as decision nodes, forks and joins.
The result is very intelligible, similar to Activity Diagrams but with a slant of BPMN and a unique ability to blend structural and functional modeling.
Interestingly I've just found this paper that compares BPMN notation with UML/SysML Activity Diagrams:
http://www.workflowpatterns.com/vendors/documentation/BPMN_wfh.pdf
