Note documentation of window-configuration-to-register
(window-configuration-to-register REGISTER)
Probably introduced at or before Emacs version 19.20.
Store the window configuration of the selected frame in register REGISTER.
Use C-x r j to restore the configuration.
Argument is a character, naming the register.
At https://github.com/bling/fzf.el/blob/521d18933cb586337c4e34281bdc71ac07202c98/fzf.el#L110
Note documentation of
window-configuration-to-register
See also https://github.com/emacs-evil/evil/pull/1212