(Sometimes these are called transfer modes, but for clarity we'll call them connection modes.)
The connection mode is established at the very beginning of any FTP transaction.
This mode is dependent on what type of file structure the server is using. Most modern
systems use a Binary file structure. Some older Unix or Mainframe
Systems may use an ASCII file structure. In most cases this mode
is decided by the server and your client software will automatically detect which of
these two modes to use.