For questions related to saving and restoring Emacs session data with `desktop-save-mode` and related functions.
desktop.el is an Emacs library that provides tools for managing your Emacs session, so that the state it is in when you close Emacs is restored the next time you start it.
- some global variables
- the list of buffers with associated files. For each buffer also
- the major mode
- the default directory
- the point
- the mark & mark-active
- some local variables
- frame and window configuration
This is documented in the Emacs manual, "Saving Emacs Sessions"