Normally, each frame has its own minibuffer window at the bottom, which
is used whenever that frame is selected. You can get that window with
the function minibuffer-window (see Minibuffer Windows).
However, you can also create a frame without a minibuffer. Such a frame
must use the minibuffer window of some other frame. That other frame
will serve as surrogate minibuffer frame for this frame and cannot
be deleted via delete-frame (see Deleting Frames) as long as
this frame is live.
When you create the frame, you can explicitly specify its minibuffer
window (in some other frame) with the minibuffer frame parameter
(see Buffer Parameters). If you don’t, then the minibuffer is found
in the frame which is the value of the variable
default-minibuffer-frame. Its 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
minibuffer-auto-raise to t. See Raising, Lowering and Restacking Frames.
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.