A broadcast sends one message to many acts at once: useful for last-minute cover, or letting a group know about work coming up.
You can target by act type, so a message about a trad session doesn't land with every DJ on your roster.
Broadcasts appear in each act's message thread with you, so a reply comes back as a normal conversation rather than disappearing.
Use them sparingly. An agency that broadcasts constantly gets ignored, and the acts you most want to reach are the ones who'll mute you first.