The scon -a means to do the command in all connections.
Using $! makes mirc process $server on each connection instead of processing it immediately and echoing that server as if all of the servers had that name.
So, this would send a message to all servers saying Hello from the server you sent the command from.
scon -a amsg Hello from everyone on $server.
And this one would say Hello to everyone on $server but the server name will be the server the message is being sent to and not the server the command was sent from.
scon -a amsg Hello to everyone on $!server.
Hope this is what you needed.