Stand In
Stand In mode first routes a request to the virtual service (the same as Most Efficient mode). However, if the virtual service does not have a response, the request is then automatically routed to the live system. You can only enable Stand In mode for a virtual service with a Live Invocation step. Stand In mode does not do any special tracking. It simply allows for a virtual service to fall back on the live service.
Failover
Failover mode first routes a request to the live system (the same as Live System mode). However, if the live system does not have a response, the request is then automatically routed to the virtual service. This mode is the opposite of Stand In mode. You can only enable Failover mode for a virtual service with a Live Invocation step. Failover mode uses the service image to determine a response if the Live Invocation step actually fails (as might happen if the live system were not available).