Architecting and designing : Business Process Analysis (BPA) : BPMN : Introduction to BPMN : Groups
  
Groups
The Group symbol provides a visual mechanism to group elements of a process flow informally. Groups are often used to highlight certain sections of a diagram without adding additional constraints as, for example, a subprocess would. The grouping is for documentation or analysis purposes only: the group of activities formed does not affect the Sequence Flow.
See also Groups on business process diagrams.
See also
Introduction to BPMN