Chapter 2 Advanced Routing for FortiOS 5.0 : Border Gateway Protocol (BGP) : BGP background and concepts : How BGP works : Decision phase 3
  
Decision phase 3
Phase 3 is route distribution or dissemination. This is the process of deciding which routes the router will advertise. If there is any route aggregation or summarizing, it happens here. Also any route filtering from route maps happens here.
Once Phase 3 is complete, an update can be sent out to update the neighbor of new routes.