As an Artifact, a Group is not an activity or any Flow Object, and, therefore, cannot connect to Sequence Flow or Message Flow. In addition, Groups are not constrained by restrictions of Pools and Lanes. This means that a Group can stretch across the boundaries of a Pool to surround Diagram elements, often to identify activities that exist within a distributed business-to-business transaction.
Groups are often used to highlight certain sections of a Diagram without adding additional constraints for performance– as a Sub-Process would. The highlighted (grouped) section of the Diagram can be separated for reporting and analysis purposes. Groups do not affect the flow of the Process.