The mechanics of running an application in Legion will, of course, vary widely from one application to another. In general, however, you will want to compile, register, and run individual applications with commands written specifically for that purpose. For example, MPI applications use legion_mpi_register and legion_mpi_run, and PVM applications use legion_pvm_register. Some commands may require you to select a group of hosts that the application should use before running the application, others may just ask how many hosts should be used.
You may want to arrange parts of your context space or to organize certain resources around the needs of specific applications (a special context for host objects that can be used for a specific MPI application, for instance).
Once you're ready to start an application, you can view its output with a tty object (a special Legion object that monitors and directs output from a shell's processes: see page 75). The tty object can send your output to a file or a window.