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

// 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 $name, $icon in $fa-icons {
  .#{$fa-css-prefix}-#{$name} {
    #{$fa-icon-property}: unquote("\"#{ $icon }\"");
  }
}