In general, here are 5 important facts about arguments and mixins:

1. Mixins can take multiple arguments.
2. Sass allows you to explicitly define each argument in your @include statement.
3. When values are explicitly specified you can send them out of order.
4. If a mixin definition has a combination of arguments with and without a default value, you should define the ones with no default value first.
5. Mixins can be nested.

Here are some concrete examples of the rules:

@mixin dashed-border($width, $color: #FFF) {
  border: {
     color: $color;
     width: $width;
     style: dashed;
  }
}

span { //only passes non-default argument
    @include dashed-border(3px);
}

p { //passes both arguments
    @include dashed-border(3px, green);
}

div { //passes out of order but explicitly defined
   @include dashed-border(color: purple, width: 5px); 
}

In the example above, the color of the border of span elements would be white, the border of paragraph elements would be green, while the div elements would have a thicker purple border.