The SET LOG command determines only the name of a log file; it
    does not cause the debugger to create or write to the specified
    file. The SET OUTPUT LOG command accomplishes that.

    If you entered a SET OUTPUT LOG command but no SET LOG command,
    the debugger writes to the file SYS$DISK:[]DEBUG.LOG by default.

    If the debugger is writing to a log file and you specify another
    log file with the SET LOG command, the debugger closes the former
    file and begins writing to the file specified in the SET LOG
    command.

    Related commands:

       SET OUTPUT LOG
       SET OUTPUT SCREEN_LOG
       SHOW LOG