You can select the source equipment for a system using the Select Equipment tool available from the system editor. The source equipment will act as the end of the critical path. If the source equipment is not selected, Revit will determine the end of the critical path on its own.
When more than one system is associated with the selected component, the available systems are listed in the System Selector dropdown.
All except the components in the selected system are dimmed in the view, and the Edit Duct System or Edit Pipe System ribbon commands display.
The selected component is listed in the System Equipment parameter in the Properties palette.