Font-Awesome/js-packages/@fortawesome/fontawesome-free/less/_icons.less

// specific icon class definition
// -------------------------

/* Font Awesome uses the Unicode Private Use Area (PUA) to ensure screen
   readers do not read off random characters that represent icons */

each(.fa-icons(), {
  .@{fa-css-prefix}-@{key} {
    @{fa-icon-property}: @value;
    @{fa-duotone-icon-property}: %("%s%s", @value, @value);
  }
});