Business Purpose
Suppliers with the Standard Supplier role can participate in sourcing projects. If you would like to send the sourcing project to potential suppliers, the suppliers need a separate business partner role definition: Bidder. Assigning this business partner role will allow you to invite potential suppliers as bidders to sourcing projects.
Note
The supplier needs to have the Standard Supplier role before awarding process and creating operational documents like central contracts or purchase orders.
Technical Explanation
The Bidder role needs to be activated in business partner role customizing.