A sub-process is a group of activities that are performed as part of a larger process or choreography. It is compound in the sense that it can be broken down into finer details (a process or choreography) through a set of sub-activities. 
 Silver, B. (2009). BPMN Method and Style: A levels-based methodology for BPM process modeling and improvement using BPMN 2.0. In CodyCassidy Press US.