System logical names can be defined in SYS$MANAGER:
    NET$LOGICALS.COM which allow you to modify certain network
    defaults.  If you don't have a net$logicals procedure on
    your node, you can create one by copying the SYS$MANAGER:
    NET$LOGICALS.TEMPLATE file.  Here are some of those logicals:

    NET$entity_name_STARTUP   Specifies a logical name that points
                              to an alternate file location for
                              that entity's startup script.

    NET$IGNORE_DECNET         If set to true, the network is not
                              started.  If not defined or set to
                              false, the network starts normally.

    NET$IGNORE_EVD            If set to true, the event dispatcher
                              is not started.  Use of this logical
                              is unsupported.

    NET$STARTUP_MOP           If set to true, MOP starts.  If not
                              defined or set to false, MOP is not
                              started.  Note that in order for MOP to
                              start correctly, the MOP startup scripts
                              must exist on the system, so MOP must
                              have been configured using net$configure.
                              Simply defining this logical is not
                              enough to create the MOP entity.

    NET$STARTUP_QUIET_NCL     If defined to false, NCL output is
                              maximized during network startup.

    NET$NOISY_SHUTDOWN        If defined to true, NCL output is
                              displayed during shutdown.

    NCL$ENVIRONMENT           Defining to "NoBackTrans" prevents NCL
                              from translating addresses to fullnames.

    NET$APPLICATION_SHUTDOWN  Defined to point to a site-specific
                              procedure which will shutdown network
                              applications before DECnet is shutdown.

    NET$LOCAL_NAME_DATABASE   Defined to point to a LOCAL database
                              file if that file is located somewhere
                              other than SYS$SYSTEM.

    NET$LOCAL_CLOSE           If defined to 1, the
                              NET$LOCAL_NAME_DATABASE file will be
                              closed after each reference.

    In addition, logicals can be defined in NET$LOGICALS.COM to modify
    default quotas for the NET$ACP, NET$EVD or NET$MOP processes or to
    assign the output for those processes to certain files.  Care must
    be taken in modifying these defaults.

    Please refer to the .TEMPLATE file for more information.