The billboard messaging extension of dm3 describes a communication scheme for a public messaging application, where many senders can send messages to one recipient and all messages are publicly viewable (billboard). The communication is only in one direction (from the senders to the billboard). There is no reply from the billboard to the sender.
Billboard messaging is used e.g. as a public comment service for public content or for displaying live public discussions in a streaming application, or similar.
The senders are not anonymous but identify themselves by their dm3 profile (wallet address).