How to understand the "r-t" in ubus monitor

in ubus mointor, r t means send or receive
-M <r|t> (for monitor): only capture received or transmitted traffic

suppose there is a ubus client , ubus server,
if client send message to server, ubusd receive a msg , then send msg to server
there should be two same msg :"<- msg" "-> msg" ??

i also dont understand the code in cli

printf("%s %08x #%08x %14s: %s\n", send ? "->" : "<-", client, peer, ubus_cli_msg_type(type), data);

what is the difference between 'client' and 'peer'