Hi Nitin
BP is a Business Partner, therefore an Activities the user performs can be linked into a Partner Detemination Procedure.
A User, indicates, you're only assigning at a User, level and then no recording at BP level.
As far as I'm away you must assign someone to a Role, there is no Default if the user is not assigned, In fact you'll get an error message if the user is not assigned when they try to login to the WEBUI
Regards
Arden