To copy a file from one remote host to another:
rcp goya:tmp/vi.ref pollux:editors/vi.ref
This copies the file vi.ref from your home directory on the host "goya" to the subdirectory editors in your home directory on the other remote host "pollux".
To copy a file between one remote host on which you have a different username, and another remote host on which you have the same username as on the local host (the one you are giving the command from):
rcp festival:README jamie@scorpio:README
This copies the file README from the home directory of your account on the remote host "festival" to the home directory of your account on the remote host "scorpio". On this host you have the username "jamie".
To copy a file between remote hosts, on each of which you have a different username to that on the local host (the host you are giving the command from):
rcp joe@ubik:News/funnies helper@pollux:new_jokes
This copies the file funnies from the subdirectory News in your home directory on the host "ubik". On this host you have the username of "joe".
The copy is placed in the file new_jokes in your home directory on the host "pollux". On this system you have the username of "helper".