The default behaviour of uwl is when user logs onto the UWL iView,the UWL tries to log that user onto all registered back end systems and try to retrive the workitem through the connector created in Universal worklist administration with the system alias name.
Only one UWL connector per back-end system is supported.In some case when you have to fetch items from many backend systems and the portal end user not exist in all the backend system,it will throw error message that user not exist or related error in the log.
In large landscapes the portal load increase when user login from portal and access uwl iview.Also the enduser accessing uwl will get an error messages from those backend systems where the logged in portal user has no userid in backend system.
The best way to restrict the backend system for the enduser to avoid load and error messages through the below 2 connector property value:
*User Roles
*Configuration groups
Kindly refer the below help link to know more about these UWL connector properties.
https://help.sap.com/saphelp_nw74/helpdata/en/4b/0497c463c12b5fe10000000a42189c/content.htm