Desktop Save Mode is a feature to save the state of Emacs from one session to another. The user-level commands for using Desktop Save Mode are described in the GNU Emacs Manual (see Saving Emacs Sessions in the GNU Emacs Manual). Modes whose buffers visit a file, don’t have to do anything to use this feature.
For buffers not visiting a file to have their state saved, the major
mode must bind the buffer local variable desktop-save-buffer to
a non-nil value.
If this buffer-local variable is non-nil, the buffer will have
its state saved in the desktop file at desktop save. If the value is
a function, it is called at desktop save with argument
desktop-dirname, and its value is saved in the desktop file along
with the state of the buffer for which it was called. When file names
are returned as part of the auxiliary information, they should be
formatted using the call
(desktop-file-name file-name desktop-dirname)
For buffers not visiting a file to be restored, the major mode must
define a function to do the job, and that function must be listed in
the alist desktop-buffer-mode-handlers.
Alist with elements
(major-mode . restore-buffer-function)
The function restore-buffer-function will be called with argument list
(buffer-file-name buffer-name desktop-buffer-misc)
and it should return the restored buffer.
Here desktop-buffer-misc is the value returned by the function
optionally bound to desktop-save-buffer.