Broadcast MeshMS allows a user of the Serval Mesh (app for Android) to send a MeshMS text message to all other Mesh network users. It does this by setting the recipient field of the Rhizome manifest to the broadcast address, and by the Rhizome transport and MeshMS implementations handling this address specially.
This feature was hastily implemented for the Shuttleworth Summit in Berlin in mid 2012. The initial implementation had the problem that all recipients of a broadcast message would send an ACK back to the sender, just as they would for any received MeshMS. In a large Mesh network, this would cause a storm of unwanted Rhizome bundles, congesting the network and effectively turning a Broadcast MeshMS into a DDoS attack.