Every frame has a name parameter; this serves as the default for the frame title which window systems typically display at the top of the frame. You can specify a name explicitly by setting the name frame property.
Normally you don’t specify the name explicitly, and Emacs computes the frame name automatically based on a template stored in the variable frame-title-format. Emacs recomputes the name each time the frame is redisplayed.
This variable specifies how to compute a name for a frame when you have not explicitly specified one. The variable’s value is actually a mode line construct, just like mode-line-format, except that the ‘%c’, ‘%C’, and ‘%l’ constructs are ignored. See Mode Line Data.
This variable specifies how to compute the name for an iconified frame, when you have not explicitly specified the frame title. This title appears in the icon itself.
This variable is set automatically by Emacs. Its value is t when there are two or more frames (not counting minibuffer-only frames or invisible frames). The default value of frame-title-format uses multiple-frames so as to put the buffer name in the frame title only when there is more than one frame.
The value of this variable is not guaranteed to be accurate except while processing frame-title-format or icon-title-format.
Copyright © 1990-1996, 1998-2019 Free Software Foundation, Inc.
Licensed under the GNU GPL license.
https://www.gnu.org/software/emacs/manual/html_node/elisp/Frame-Titles.html