Normally, each frame has its own minibuffer window at the bottom, which
is used whenever that frame is selected. If the frame has a minibuffer,
you can get it with
minibuffer-window (see Definition of minibuffer-window).
However, you can also create a frame with no minibuffer. Such a frame
must use the minibuffer window of some other frame. When you create the
frame, you can explicitly specify the minibuffer window to use (in some
other frame). If you don’t, then the minibuffer is found in the frame
which is the value of the variable
value should be a frame that does have a minibuffer.
If you use a minibuffer-only frame, you might want that frame to raise
when you enter the minibuffer. If so, set the variable
t. See Raising and Lowering.
This variable specifies the frame to use for the minibuffer window, by default. It does not affect existing frames. It is always local to the current terminal and cannot be buffer-local. See Multiple Terminals.