What is set mouse = a for in nvim?

:set mouse=a the 'a' means all vim modes: visual, normal, insert, command line.

This is the help of vim:

'mouse'                 string  (default "", "a" for GUI, MS-DOS and Win32
                                        set to "a" or "nvi" in defaults.vim)
                        global
        Enable the use of the mouse.  Works for most