If the logical name DBG$INIT is defined at the start of a
    debugging session, then the file that it translates to is used
    as an initialization file. The commands in the file are executed
    as if the file had been called with the @ (Execute Procedure)
    command. This is useful if there is a particular set of commands
    that you always execute when you start up the debugger, for
    example to specify a source directory search list, enable screen
    mode, log the session.

    Example:

    $ CREATE MY_DEBUG.COM
    SET SOURCE SYS$DISK:[],SRC$
    SET MODE SCREEN
    SET STEP SILENT
         . . .
    $ DEFINE DBG$INIT [JONES.CMD]DEBUG_INIT.COM