The paragraph says:
When a client sends a broadcast message to the server,
from_usernamemust be an empty string. When the server sends a broadcast
message to clients,from_usernamewill be the username of the sender of the
broadcast message, or an empty string if the broadcast message comes from
the server itself.
However, in my opinion, when a client sends a broadcast message to the server, from_username should be the client’s actual username—not an empty string.