From GridLAB-D Wiki
The --server command line option instructs GridLAB-D to run in server mode with an extra thread to service HTTP requests on the server port number specified by the server_portnum global variable. Once server mode is started, incoming messages on the server port will be handled as HTTP request. Response can be either in HTML, XML, or data files such as images, CSV files, etc., depending on the type of request made.