min-width for mq-w() and max-height for mq-h() are default value. We can use number [320px], variable[$md] or operator[ $md - .02 ] as parameter of mq-w() or mq-h(). If we want explicit max-width and min-height for mq-w() and mq-h() respectively, we need to set value [$limit : max or min] as second parameter of that mixin.]

BS4 breakpoints

### breakpoints(xs: 0, sm: 576px, md: 768px, lg: 992px, xl: 1200px) ###

# Variables
$xs: 0px;
$sm: 576px;
$md: 768px;
$lg: 992px;
$xl: 1200px;

# Width
@mixin mq-w($val, $limit: min ){
  @media(#{$limit}-width: $val){
    @content;
  }
}

# Height
@mixin mq-h($val, $limit: max ){
  @media(#{$limit}-height: $val){
    @content;
  }
}