server-window

server-window is a variable defined in `server.el'.
Its value is

nil

Documentation:
Specification of the window to use for selecting Emacs server buffers.
If nil, use the selected window.
If it is a function, it should take one argument (a buffer) and
display and select it. A common value is `pop-to-buffer'.
If it is a window, use that.
If it is a frame, use the frame's selected window.

It is not meaningful to set this to a specific frame or window with Custom.
Only programs can do so.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.