5.2.4 The Alt modifier and editor bindings

In Microsoft Windows emulation on Microsoft Windows, keystrokes with the Alt modifier key are used by the system to activate the menu bar. Therefore these keystrokes, for example Alt+A and Alt+Ctrl+A are not available to the editor.

Windows accelerators always take precedence over editor key bindings, so in Emacs emulation the Alt modifier key only acts as Meta though keystrokes with Alt if there is no accelerator which matches.

On Cocoa, the preference for the Meta key affects the operation of menu accelerators (shortcuts). If Command is used as Meta, then it will not be available for use as an acclerator.

LispWorks Editor User Guide (Macintosh version) - 14 Mar 2008