is_macIE global variables to identify Microsoft Internet Explorer as part of browser detection in [wordpress root folder]/wp-includes/vars.php. They are set by looking for strings MSIE and Win or Mac in the HTTP_USER_AGENT server variable. It also sets another variable
is_IE which is set to true if the browser is Microsoft Internet Explorer on Microsoft Windows or Macs.
Plugin and theme authors can use these global variables to identify browsers to handle any browser specific features.
Back to full list of global variables.