Top: Start Asynchronous Call - Loads a VI, starts running the VI, and then waits for it to complete. The SAC returns as soon as the VI has started to run. The Wait On Asynchronous Call is what waits for the VI to complete and return its outputs.
Middle: Call By Reference - Loads a VI and runs it. The CBR will not return until the called VI is complete.
Bottom: Run VI Invoke Node - Loads a VI and runs it. If you wire FALSE to the Wait Until Done input, the VI runs asynchronously. If you wire a TRUE to the Wait Until Done input, the invoke node will not return until the VI is complete. You do not have a direct way of getting the results once the VI is complete.