Hi RB,
The purpose of the "Associated L3 Outs" field under a bridge domain is to add the BD subnets to the outbound prefix-list(s) for the listed L3Out(s). The default setting of "Match Prefix AND Routing Policy" tells ACI that you want to allow those BD subnets in addition to whatever is defined in the route-map to be advertised. You can forgo the BD-to-L3out association, and instead use only the manually defined entries in the route-map for the outbound prefix-list by using the "Match Routing Policy Only" option. Using a route-map will allow you more granular control, but otherwise it's just a personal choice which method you use.
