If you have configured multiple transport domains (see Transport Management System - Setting Up a System Landscape) and want to make transports between systems in different domains, you can use domain links to link the two domains.
There must be a permanent network connection between the systems in the two domains, similar to the connection between systems within the same domain.
If you cannot operate a permanent RFC connection between systems in the two domains, you can use external systems to make transports between the two.