The option -S <service> is used to specify the service (port) on which SAProuter runs (default 3299). SAProuter can, for example, be started on any other service:
Example
saprouter -r-S 4444 (IBM i: saprouter '-r -S 4444') starts SAProuter on the local host on service 4444.
If you then want to administer this SAProuter, the service obviously has to be specified.