The depicted ad-hoc Sub-Process (recognisable by its dashed border and absence of sequence flows between contained activities) allows its contained activities to be executed “in any order, or even in parallel,” as determined at runtime. BPMN 2.0 specifies: “The order of execution of the contained activities is undefined” (OMG BPMN 2.0, §10.2.2, p. 174). Therefore activities C and D have no predefined ordering; whichever starts first is implementation-dependent.

A. A predefined sequence putting C before D is not shown.

B. No sequence flow or rule places D before C.

C. Parallel execution is only one possible runtime behaviour; the specification states the order is undefined, not necessarily parallel.

1. OMG BPMN 2.0.2, formal/2013-12-09, §10.2.2 “Ad-Hoc Sub-Process,” p. 174.

2. van der Aalst, W., “Process Mining,” Springer, 2016, §3.4.3, pp. 82-83 (ad-hoc semantics).