bootstrap/scss/utilities/_mixin.scss

$utilities-map: build-utilities-map(); // stylelint-disable-line scss/dollar-variable-default

@mixin utl($class) {
  @if map-has-key($utilities-map, $class) {
    $definition: map-get($utilities-map, $class);
    $breakpoint: map-get($definition, breakpoint);
    @if $breakpoint != null {
      @include media-breakpoint-up($breakpoint) {
        @each $property in map-get($definition, properties) {
          #{$property}: map-get($definition, value);
        }
      }
    }
    @else {
      @each $property in map-get($definition, properties) {
        #{$property}: map-get($definition, value);
      }
    }
  }
  @else {
    @debug "Unknown utility class " + $class;
  }
}