mjhjmtu

According to man tmux to check the launch option:

-C: Start in control mode (see the CONTROL MODE section). Given
twice (-CC) disables echo.

Then in the control mode section of the man tmux, there is the following description:

CONTROL MODE
 tmux offers a textual interface called control mode. This allows
 applications to communicate with tmux using a simple text-only protocol.

 In control mode, a client sends tmux commands or command sequences
 terminated by newlines on standard input. Each command will produce one
 block of output on standard output. An output block consists of a %begin
 line followed by the output (which may be empty). The output block ends
 with a %end or %error. %begin and matching %end or %error have two
 arguments: an integer time (as seconds from epoch) and command number.
 For example:

 %begin 1363006971 2
 0: ksh* (1 panes) [80x24] [layout b25f,80x24,0,0,2] @2 (active)
 %end 1363006971 2

 The refresh-client -C command may be used to set the size of a client in
 control mode.

 In control mode, tmux outputs notifications. A notification will never
 occur inside an output block.

I'm not sure what it means, but at least as far as I try a few commands and try to see the looks and feels of it via (tmux -CC), it looks like the same as when I launch via tmux new-session.

So what is the "control mode" and what makes it different from the normal mode?

EDIT

I found that the session and the window that was launched via the control mode (-CC) does not react to the keyboard shortcut of the tmux commands, such as window split. So what is the point of using the control mode in the first place?

The interesting aspect of control mode is that
you can write a background process that listens to a real tmux process. It gets notifications of
things happening in the real tmux, and it can then send
commands. If you use 2
terminals and run a normal session in one

tmux new -s mysession

and in the other

tmux -C attach -t mysession

then when you split windows, add new ones, or close them in the normal tmux
you will get lines like

%layout-change @2 91a8,80x23,0,0[80x11,0,0,5,80x11,0,12,7]
%window-add @3
%window-close @1

in the control tmux, to which you can react by writing a program. To help
there is a python library to exploit
this mechanism. See the examples there.

You are seeing similar results from tmux -CC as you would from tmux new-session because you didn't specify a command, so tmux uses the default, which is new-session:

command [flags]

This specifies one of a set of commands used to control tmux, as described in the following sections. If no commands are specified, the new-session command is assumed.

Adding -CC doesn't appear to change that. To control the existing session, you should attach to it in control mode:

tmux -C attach