Actually,
<A-K> and <A-S-K> are the same.  Instead of "A" you can use "M".  If you have
an actual Meta modifier key, please see |:map-meta-keys|.

In the GUI Nvim handles the |ALT| key itself, thus mapping keys with ALT
should always work.  But in a terminal Nvim gets a sequence of bytes and has
to figure out whether ALT was pressed.  Terminals may use ESC to indicate that
ALT was pressed.  If ESC is followed by a {key} within 'ttimeoutlen'
milliseconds, the ESC is interpreted as:
        <ALT-{key}>
otherwise it is interpreted as two key presses:
        <ESC> {key}

1.11 MAPPING META-KEYS					*:map-meta-keys*

Mapping keys with the Meta modifier works very similar to using the Alt key.
What key on your keyboard produces the Meta modifier depends on your keyboard
and configuration.

Note that mapping <M-a> actually is for using the Alt key.  That can be
confusing!  It cannot be changed, it would not be backwards compatible.

For the Meta modifier the "T" character is used.  For example, to map Meta-b
in Insert mode: >
	:imap <T-b> terrible

1.12 MAPPING SUPER-KEYS or COMMAND-KEYS		*:map-super-keys* *:map-cmd-key*

The Super / Command modifier is available if the terminal or GUI supports it.
The character "D" is used for the Super / Command modifier.

For example, to map Command-b in Insert mode: >
	:imap <D-b> barritone

1.13 MAPPING AN OPERATOR				*:map-operator*

An operator is used before a {motion} command.  To define your own operator
you must create a mapping that first sets the 'operatorfunc' option and then
invoke the |g@| operator.  After the user types the {motion} command the
specified function will be called.

							*g@* *E774* *E775*
g@{motion}		Call the function set by the 'operatorfunc' option.
			The '[ mark is positioned at the start of the text
			moved over by {motion}, the '] mark on the last
			character of the text.
			The function is called with one String argument:
			    "line"	{motion} was |linewise|
			    "char"	{motion} was |charwise|
			    "block"	{motion} was |blockwise-visual|
			The type can be forced, see |forced-motion|.

Here is an example that counts the number of spaces with <F4>: >

	nnoremap <expr> <F4> CountSpaces()
	xnoremap <expr> <F4> CountSpaces()
	" doubling <F4> works on a line
	nnoremap <expr> <F4><F4> CountSpaces() .. '_'

	function CountSpaces(context = {}, type = '') abort
	  if a:type == ''
	    let context = #{
	      \ dot_command: v:false,
	      \ extend_block: '',
	      \ virtualedit: [&l:virtualedit, &g:virtualedit],
	      \ }
	    let &operatorfunc = function('CountSpaces', [context])
	    set virtualedit=block
	    return 'g@'
	  endif

	  let save = #{
	    \ clipboard: &clipboard,
	    \ selection: &selection,
	    \ virtualedit: [&l:virtualedit, &g:virtualedit],
	    \ register: getreginfo('"'),
	    \ visual_marks: [getpos("'<"), getpos("'>")],
	    \ }

	  try
	    set clipboard= selection=inclusive virtualedit=
	    let commands = #{
	      \ line: "'[V']",
	      \ char: "`[v`]",
	      \ block: "`[\<C-V>`]",
	      \ }[a:type]
	    let [_, _, col, off] = getpos("']")
	    if off != 0
	      let vcol = getline("'[")->strpart(0, col + off)->strdisplaywidth()
	      if vcol >= [line("'["), '$']->virtcol() - 1
	        let a:context.extend_block = '$'
	      else
	        let a:context.extend_block = vcol .. '|'
	      endif
	    endif
	    if a:context.extend_block != ''
	      let commands ..= 'oO' .. a:context.extend_block
	    endif
	    let commands ..= 'y'
	    execute 'silent noautocmd keepjumps normal! ' .. commands
	    echomsg getreg('"')->count(' ')
	  finally
	    call setreg('"', save.register)
	    call setpos("'<", save.visual_marks[0])
	    call setpos("'>", save.visual_marks[1])
	    let &clipboard = save.clipboard
	    let &selection = save.selection
	    let [&l:virtualedit, &g:virtualedit] = get(a:context.dot_command ? save : a:context, 'virtualedit')
	    let a:context.dot_command = v:true
	  endtry
	endfunction

An <expr> mapping is used