Hello Raju,
in case you talking about the user for the Entitlement System - yes that is a generic user but thats a user that your actual users will never see. The user will only be used for communication between the systems.
In regards to your scenario:
The BI platform has a concept of user aliases and a user could potentially logon with USERA in Enterprise Authentication but have USERB for SAP authentication setup as a user alias.
In general you will have to get the SAP authentication setup as otherwise you would not have the data level security towards the BW system.
I would suggest you sit down with the folks that have configured the Siteminder part and explain the issue as I am sure they already have solved a similar scenario
regards
Ingo Hilgefort, Visual BI