Use the System Selector, on each of the Systems/Circuits tabs for a selected element, to select a system associated with a connector on the element.
After systems are created, the System Selector on each of the Systems/Circuits tabs displays a list of all the systems associated with that domain for the selected component. You can use the System Selector to select a system as an alternative to positioning the cursor over the element, pressing Tab until the desired system displays in the status bar, and clicking to select the system.
For example, if you create systems using the air handling unit that is included in the default template, tabs display on the ribbon for Piping, Electrical, and Duct systems and each one includes the System Selector.