The argument field is a Telnet string identifying the user.
The user identification is that which is required by the
server for access to its file system. This command will
normally be the first command transmitted by the user after
the control connections are made (some servers may require
this). Additional identification information in the form of
a password and/or an account command may also be required by
some servers. Servers may allow a new USER command to be
entered at any point in order to change the access control
and/or accounting information. This has the effect of
flushing any user, password, and account information already
supplied and beginning the login sequence again. All
transfer parameters are unchanged and any file transfer in
progress is completed under the old access control
parameters.