The VisIt Component Launcher (VCL) is a small component that VisIt launches on remote computers when running in distributed mode. The VCL is launched first when VisIt needs to launch its mdserver or compute engine on a remote computer. When VisIt wants to run components on a remote computer, secure-shell (SSH) is used and depending on how your settings are configured, you may have to enter a password when invoking commands on remote computers via ssh. To avoid users having to frequently enter passwords when VisIt needs to launch a remote component, the VCL is launched on the remote computer first. This means that if you need to enter a password, you will do so only once. Subsequent requests for launching VisIt components are routed through VCL, with which the local VisIt viewer maintains a connection.
In addition to spawning processes for VisIt on remote computers, VCL forwards console output from those program back to the local viewer process, which then prints that output back to the console. Forwarding console output can be especially helpful for seeing batch system arguments used to run VisIt on a remote system. Besides launching processes and forwarding their output, VCL can also create a socket bridge through SSH that enables remote components to connect back to your local computer in more restrictive networking environments.